VSD energy savings 18

[mfqd13]

Hi,

I would like to perform some detailed calculations to determine the energy saving in the appliance of a VSD (variable speed drive) in electric motors mainly for pumps.
I searched in many references and there are some explanations, but i didn't find yet one that suits for my intentios. Complete enough...
So, my base point is that i can only measure in site the power consumption of the motor and with this i would like to perform some calculations to estimate the energy saving after apply a VSD.

Can anyone help me?

 

[stanier]

How many myths have morphed into legends to be considered facts by the unitiated?

If anyone is in Sydney Australia on Tursday 18th february I shall be presenting a paper to a joint meeting of ASME/ Engineers Australia/IMechE on this topic. Address is engineers Australia 8 Thomas St Chatswood NSW 2067. No charge refreshments served from 1730 hours. Dinner afterwards.

 

[stevenal]

I would suggest using kWh/gal rather than kW/gal.

 

[jonr12]

I think I may have found another myth. This discussion is getting long but is quite good and is helping me a lot. I don't know if I should start another discussion with this but it is all related so I will just keep going here. Please let me know if I need to start another discussion.

Using the example of needing 80% flow as described by BigInch, I went back to my pump curve. At 100% flow this pump is using 9.2 HP. Because of the second affinity law I would only be able to slow this pump down by 4% and still produce the head required. 4% reduction in speed gives an 8% head reduction and 12% reduction in horse power. 12% reduction in horse power brings this pump down from 9.2 HP to 8.1 HP.

I can't figure out how to paste the curves. However, the full speed curve shows this pump will reduce to the same 8.1 HP load without decreasing the speed. I could simply let the system head or pump discharge pressure increase by 8%, which is only 12 PSI, or I could use a valve to maintain the same discharge pressure. Simply reducing the flow by 20% either way, seems to require the same horse power as if slowing the RPM with a VSD/VFD.

Because the second affinity law says I can only slow the RPM of this pump by 4% and still produce the head required, the third affinity law says that 8.1 HP should be the lowest power possible at any flow rate. However, I plugged this minimum speed into the curve and can see that the actual power required will drop even lower than the third affinity law states, as flow continues to decrease. So there is something wrong there? I have read several places where the affinity law assumes two points of a curve that have the same efficiency. How can this be if the efficiency is different at different flow rates? Does that mean that the affinity law only works when reducing the head, not the flow? Otherwise why does the affinity law calculations not match the horse power from the curve?

Anyway using the curve at the minimum speed required, I can see that at 50% flow the power required drops from 9.2 to 6.1 HP. However, the full speed curve shows the horse power drop from 9.2 to 6.5 HP by simply decreasing the flow, not the RPM. At 50% flow, that is only 6% less power being used by varying the RPM, than by just riding the curve or restricting with a valve. If you can say that using a VSD to reduce the horse power from 9.2 to 6.1 is saving energy, then using a valve to reduce the horse power from 9.2 to 6.5 is also saving energy. Then if you take into account that at full speed the losses from a VSD adds 5% to the energy consumption, and a fully open valve does not, it seems there is little if any difference between using a valve and a VSD throughout the entire flow range. So I guess it is also a myth that valves burn energy?

I was under the impression, as BigInch stated, that using a control valve would cause the pump to run at 100% BEP and 100% head inside the pump. However calculations and the curves show that using a control valve reduces the power required almost exactly the same as using a VSD to vary the RPM. The only thing I can't see from the curve is the loss of motor efficiency at partial load. Is this efficiency the same for a motor that is slowed down to partial load, as compared to a motor that is still running at full RPM and just drawing a partial load? If not, then maybe this is where my problem lies. Otherwise I would say that replacing a valve with a VSD to save energy is another "myth-application".


Still nothing produces as many gallons per horse power as letting the pump run at maximum flow and filling a big tank. Second to this a two pump system is still more efficient than using a VSD on a single pump. Now I also believe that using a control valve on one pump, or to round the edges between a two pump system, may be just as good as using a VSD.

I wonder how many more "myths have morphed into legends to be considered facts by the un-initiated"? I also wonder how much energy the world is wasting by falling for these myths? I wish I could go to Sydney to hear you Stanier. Is there anyway you could put your paper here after you have presented it?

 

[BigInch]

Weekend. I'll get back to you on Monday.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

Jonr,

lets make a new thread. The data you have is getting hard to follow through this thread. Let's start with a fresh slate. Be sure to give flow, head, rpm, power and pump efficiency values for any point you want to talk about. I'll assume its fresh water, unless you say differently.
-------------------------------

For what its worth department.

I had a look at the Vaillencourt paper and its pretty good. I only have one disagreement. The pump is being considered alone without any pipe flow dH-Q relations. The system's static head is considered, but the speed change relationship he uses is based on the piping carrying whatever flowrate the pump makes using the total discharge head provided by the pump at any speed from minimum speed to 100% speed. While his analysis is valid at the minimum speed and at the 100% speed Q-H points, the operating point will not necessarily follow his relationship between those points. The operating point will follow the system curve between those points, which could differ significantly from his relationship. However, if the pipe and fluid parameters are not well known, its the best approximation you can make under the circumstances.

If you need to evaluate the economics of operating at intermediate points between the minimum and maximum speeds, it could be very important to use the correct system curve head values. You may do so using his method by considering each intermediate head a "design head", but you must use the system head at the correct intermediate speed, not at the 100% speed where the design head would normally be located.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

Because the second affinity law says I can only slow the RPM of this pump by 4% and still produce the head required, the third affinity law says that 8.1 HP should be the lowest power possible at any flow rate. However, I plugged this minimum speed into the curve and can see that the actual power required will drop even lower than the third affinity law states, as flow continues to decrease.


W/o checking the numbers, I'll try to answer as much as I can.
8.1 HP I think is at your min speed to still deliver flow, so anything under that is wasted energy in any case. You should turn the pump/VFD off.


I have read several places where the affinity law assumes two points of a curve that have the same efficiency. How can this be if the efficiency is different at different flow rates? Does that mean that the affinity law only works when reducing the head, not the flow? Otherwise why does the affinity law calculations not match the horse power from the curve?

Reducing speed from 100% to a lower speed is considered to carry the same efficiency at 100% speed down along with it. Thus the two points referred to are any two points along that curve.

If you can say that using a VSD to reduce the horse power from 9.2 to 6.1 is saving energy, then using a valve to reduce the horse power from 9.2 to 6.5 is also saving energy. Then if you take into account that at full speed the losses from a VSD adds 5% to the energy consumption, and a fully open valve does not, it seems there is little if any difference between using a valve and a VSD throughout the entire flow range. So I guess it is also a myth that valves burn energy?

Yes. But the supposition is that in that situation the VSD would (maybe) still save a TINY little bit more than using a valve. There is no doubt that "Valves burn energy", it isn't a "myth". VFDs also use 5% or so.

I was under the impression, as BigInch stated, that using a control valve would cause the pump to run at 100% BEP and 100% head inside the pump. However calculations and the curves show that using a control valve reduces the power required almost exactly the same as using a VSD to vary the RPM.

Yes, the pump brings up the flowrate carried in the impeller to full internal discharge pressure, but since flow cannot exit due to a valve restriction, it is recirculated and the efficiency drops according to what is shown on the pump chart.

In many situations the amount of energy saved by using valve or VFD is very small and essentially immaterial. In those cases I believe a valve is far superior due to the lesser complexity of the system required to do so. In other cases it is significant and a proper choice should be made.


The only thing I can't see from the curve is the loss of motor efficiency at partial load. Is this efficiency the same for a motor that is slowed down to partial load, as compared to a motor that is still running at full RPM and just drawing a partial load? If not, then maybe this is where my problem lies. Otherwise I would say that replacing a valve with a VSD to save energy is another "myth-application".


The loss of efficiency of the motor will never appear in the pump curve. That curve is reserved for hydraulic efficiency and shaft power to the pump only. You must adjust the power rating requirement for all additional electrical equipment yourself using the tables I provided for the motor and VFD above, whether that is at full or partial loads too.


Still nothing produces as many gallons per horse power as letting the pump run at maximum flow and filling a big tank. Second to this a two pump system is still more efficient than using a VSD on a single pump. Now I also believe that using a control valve on one pump, or to round the edges between a two pump system, may be just as good as using a VSD.

With two pumps operating within recommended ranges it may not be possible to reach all flowrates between operating scenarios of running one pump and two pumps, if your flow range is high. For example, lets say your normal allowable range is 50% to 110% for each pump. With a BEP for each pump of 100 gpm your ranges would be 50-110 and 100-220, but operating at 130 gpm would be a poor choice as that would involve running two pumps at 65 gpm each, both with poor efficiencies. A VFD and 1x200gpm pump might be a far superior choice as long as you could still make the required head at lower flows. It might be tough going down to 50 gpm. Not a MYTH. IT DEPENDS.

I wonder how many more "myths have morphed into legends to be considered facts by the un-initiated"?

As you have found out, making a proper evaluation of these types of problems is often not a simple task for experienced engineers.
Only the uneducated perpetuate and believe in myths.

I also wonder how much energy the world is wasting by falling for these myths?

Conservation and proper application is the best method to save energy.
I wish I could go to Sydney to hear you Stanier. Is there anyway you could put your paper here after you have presented it?

I'll let Stanier handle that one.


Knowledge is power^3

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[stanier]

jnor12,

The presentation will be made available on the Engineers Australia' website after the event. I will make sure I make a posting here to give a link. We post our presentations there as many members are spread across this wide land of ours and cannot get to meetings.

It is up to the older engineers to get out there and spread the word on this subject. With 35% of the world's energy involved in pumping fluids it is an important issue.

Forget the greenies and their climate change dogma there is a need to use enrgy more efficiently. There is not an environmental issue in the world that will be solved by an environmentalists. It is engineers who will be solving the challenge. Let us start now.

 

[BigInch]

Engineers can be environmentalists.


**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[LionelHutz]

There is a lot of good info here.

I (my company) sells VFDs. We do not push VFDs as a pump energy savings device because the "cookie cutter" energy savings arguements typically are not true and in many cases the complete system can use more energy once the VFD is installed.

I also agree that many, if not all, of the glossy articles I've seen claiming a VFD was a wonderful energy saver on a pump also discuss how other parts of the system were changed at the same time totally obscuring the real saving or lack of savings due to the VFD. These articles are also typically written by VFD manufacturers.

The post about certain people seeing a drop in input power (kW) believing that energy savings are occuring is true and happens more than I care to admit. There is also no point aruing with these people because they have "seen the light" - I like BigInch's approach of getting them to agree to a 2-way money contract because that would be the only way to make a point to them.

I would say that a VFD can be an effective way to achieve process control. You can maintain a certain flow rate or maintain a certain pressure.

I remember writing this quote;

"Saving energy means reducing the energy cost per gallon of water pumped, not reducing the cost per hour to run the motor turning the pump."

It was in a discussion where someone had commented that the kW of their motor was reduced with the application of a VFD. I have no idea if they had done a kWh/gallon pumped or $$/gallon pumped study but from their post I have my doubts.
The discussion was also related to an application with a fairly constant head requirement which is not a good place to try and save energy with a VFD.

A big thanks goes out to BigInch for trying to spread the truth and cut through the BS.

 

[stanier]

BigInch,

Engineers are the true environmentalists as they can actually achieve energy reductions, reduce pollution and cut out waste.

The "greenies" are like castrated tom cats on the yard post. They wail on all night about what they would like to do, but are ill equipped for the task to hand. Hence they will never solve an environmental issue but can just wail about them.

What an industrial hero Al Gore is in that he has the "greenies" chanting for nuclear power stations and uranium mines, all while he flies around in his Lear jet.

VFD salespeople love "greenies" as they are so inept that they can sell heaps of VFDs without engineering.

 

[dabluffrat]

Just thought I'd chime in on post # 50 here... be very cautious about VFD's on boiler feed applications, or any pumps operating in parallel... BFW pumps typically have flat head rise curves to shut off. The drives must be synchronized to all run at the same speed as drum levels increase or decrease. If the VFD decreases the speed on one pump, but remains the same on the other, that one pump will likely go to shut off while the other runs out past BEP.

As for the other comments about cost savings and operation, good discussions. I know pipeline companies and municipal water districts really like using VFD's in applications where the changes in system curves are severe


Did you know that 76.4% of all statistics are made up...

 

[jonr12]

"Reducing speed from 100% to a lower speed is considered to carry the same efficiency at 100% speed down along with it." BI

I don't think this is true. I have been playing with curves and even the slightest reduction in RPM also gives a reduction in efficiency. The efficiency curve moves slightly to the left as the RPM is reduced but, is no where close to the reduction of flow. The only way the efficiency stays the same is when the flow rate stays the same. So if it is true that "the affinity law assumes two points of a curve that have the same efficiency", then the affinity law only works for a reduction in head, not flow.

"You must adjust the power rating requirement for all additional electrical equipment yourself using the tables I provided for the motor and VFD above, whether that is at full or partial loads too." BI

I understand this but, it doesn't answer my question about efficiency at partial load being the same for VSD and DOL? If it is, then it doesn't make much difference for a comparison.

"Yes. But the supposition is that in that situation the VSD would (maybe) still save a TINY little bit more than using a valve. There is no doubt that "Valves burn energy", it isn't a "myth"." BI

OK, we know now that the real myth is that VFD's save energy. But "valves burning energy" is a big part of that myth. Every VFD salesman I talked to started the conversation with "valves burn energy". This led me to believe that valves were out of the question, and that VFD was my only choice. After seeing VFD's in their real light, my calculations now correctly show that "VFD's burn energy". Because of the second affinity law and the fact that my head condition does not vary, comparisons between Valve and VFD shows very little if any difference. Add back in the motor efficiency at partial load and the power the VFD itself uses, and the VFD may actually use more energy than a valve. So we shouldn't say "valves burn energy", we should say "VFD's and Valves both burn energy".

"The presentation will be made available on the Engineers Australia' website after the event. I will make sure I make a posting here to give a link." Stainer

Thanks Stainer, I will be looking forward to seeing it.

"I (my company) sells VFDs. We do not push VFDs as a pump energy savings device because the "cookie cutter" energy savings arguments typically are not true and in many cases the complete system can use more energy once the VFD is installed." LionelHutz

I appreciate such honesty from someone who sells VFD's. I have not found this to be the case with most other manufacturers and salespeople. I also believe a VFD can be an effective way to achieve process control. There are many good uses for a VFD but, claiming they save energy when they do not, has me wondering what else some manufacturers are lying about?

"Engineers are the true environmentalists as they can actually achieve energy reductions, reduce pollution and cut out waste." Stanier

I agree but, engineers who are "myth-applying" VFD's are increasing energy consumption, increasing pollution, and increasing waste. Engineers who have "seen the light", think every time they spec a VFD they are helping save the world, by saving energy and reducing green house gasses. Most get really angry when being told, this is not true, and that they do not understand what they are doing. This discussion has been an unbelievable amount of help. I just hope those who would like to keep the myth alive, do not have the connections here to get this discussion deleted. I have copied everything just in case. When I figure out how to paste a curve, I will start a new discussion. I would like opinions from others on the best way to "really" save energy in my application.

 

[BigInch]

jonr12,

"Reducing speed from 100% to a lower speed is considered to carry the same efficiency at 100% speed down along with it." BI

I don't think this is true. I have been playing with curves and even the slightest reduction in RPM also gives a reduction in efficiency.
...
So if it is true that "the affinity law assumes two points of a curve that have the same efficiency", then the affinity law only works for a reduction in head, not flow.

Jon, The efficiency curve theory has been developed from real tests. Let's don't make new ones. I don't want to turn this thread into an exercise in debunking your theories, but I guess we have to start with that one.

Efficiency tracks with speed reduction and flow quite well, also with head. It only gets a little difficult around the reverse points, but that bit can be easily ignored without excessive loss in accuracy. See this chart here by Goulds. I've circled two points to show exactly which two points the theory refers to. I'm sure you can follow how the others track too.

Also see pump specific speed charts. Specific speeds can tell you a lot about efficiencies of pumps and characteristic shapes of their cross-sections. They can also tell you about how efficiencies vary with changes in speed, rpm and head. Look at the formula, which is dependent on all three.

SpSpeed = rpm * flow_gpm^0.5 / Head_ft^0.75
Does it track with affinity laws? Yes it does.
Using a unit curve at 100% speed = 1, varying flow and head by affinity laws and then calculating SpSpeed as rpm is reduced shows that the SpSpeed won't change and therefore one wouldn't expect the efficiency to change very much either.
rpm gpm ft Specific Speed
1 1 1 1.000
0.9 0.9 0.81 1.000
0.8 0.8 0.64 1.000
0.7 0.7 0.49 1.000
0.6 0.6 0.36 1.000
0.5 0.5 0.25 1.000
0.4 0.4 0.16 1.000
0.3 0.3 0.09 1.000
0.2 0.2 0.04 1.000
0.1 0.1 0.01 1.000

I understand this but, it doesn't answer my question about efficiency at partial load being the same for VSD and DOL? If it is, then it doesn't make much difference for a comparison.

Please rephrase that question, if you want. What's DOL?
But then again, if it doesn't make a difference, maybe we should just forget it.

But "valves burning energy" is a big part of that myth. Every VFD salesman I talked to started the conversation with "valves burn energy". This led me to believe that valves were out of the question, and that VFD was my only choice.

Caveat emptor.

After seeing VFD's in their real light, my calculations now correctly show that "VFD's burn energy". Because of the second affinity law and the fact that my head condition does not vary, comparisons between Valve and VFD shows very little if any difference. Add back in the motor efficiency at partial load and the power the VFD itself uses, and the VFD may actually use more energy than a valve. So we shouldn't say "valves burn energy", we should say "VFD's and Valves both burn energy".

Yes, that conclusion has been drawn way, way up above. I've also warned specifically about boilers and other applications with high static heads. Most good pump mfgrs do too. But what we really should say is actually a question, "Which option burns LESS energy?


Why are you worried about this thread getting deleted? If we discuss this rationally with a view towards seeking real optimum solutions, there shouldn't be any problem. When one goes wild discussing thoughts that make no engineering sense and conflict with well proven methods and test data, that's when threads lose credibility, cross into rant territory... and rightfully get deleted.

Upload a curve or other document by clicking right below this where it says,

"Step 3 attachmenbt ...or upload your file
to ENGINEERING.com"


**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[ScottyUK]

BigInch,

DOL = Direct On Line, i.e. the motor is connected straight to the mains power via a contactor or circuit breaker without any fancy tricks like star-delta, VFD, soft-start, auto-transformers etc. The motor gets full voltage as soon as the contactor closes, so it accelerates quickly, draws lots of current while accelerating, and runs at one fixed speed. It is the most common type of motor starter because it is simpler and cheaper than any of the alternatives.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[BigInch]

You mean like one of those mysterious little black boxes called a switch, don't you.

Whether that makes a difference is probably dependent on the number of starts. In most cases, with anything near a properly sized storage reservoir and a properly sized pump, there shouldn't be so many to make a difference anyway. Its another band-aid to fix the symptoms of a screwed up system design, or improper operation, not cure the disease. But where it works, it works. I'd rather not muddy the water discussing those in the context of VSDs vs Control Valves, so let's make another thread out of it, if its a problem.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BigInch]

P.S.

Scotty, I hope you're working the night shift.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[stanier]

Up to now no one has mentioned the effect of the cable supplying the VFD. For a start the cable has to be shielded to prevent RFI/EMI effects. So when doing your costing allow to replace the cable for an existing motor. Given that old cables tend to harden, removing them may be a challenge without excavation, if buried, or from common cable ladders in building/plants/corridors/conduits etc.

Toshiba , a manufacturer of VFDs has some fine information for the design for VSDs.

http://www.toshont.com/vfdapp.htm

. The attached file is just one of many. these are engineering guides not sales guides. They even make sense to a mechancial engineer like me so get your electrical engineer to have a look as well.

Look at the Application Notes. nparticulalr pay attention to the voltage stress on motor windings caused by long cable lengths. Modern plants have centralised control and switchrooms. Cable lengths for motors over 60m are a problem.

 

[ScottyUK]

There are well-documented solutions to the voltage magnification problem, as there are to most of the other 'problems' identified. A good electrical engineer would be able to identify those which are a problem with the specific installation and incorporate appropriate mitigation where necessary.

On a more general note, it is unfair and unreasonable to blame poor performance of a badly designed system on the drive manufacturers who are only responsible for part of the system. What some of you are doing you're doing is akin to blaming a pump manufacturer for poor system performance caused by a bad piping design.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[mauricestoker]

Just because VFD's have been sold like snake oil and frequently misapplied does not mean they cannot save money.

I generally work on the air side, so that would be my example. But first, I'd suggest looking at ASHRAE 90.1-2004, section 6.5.4.2.1 then considering how you would control a VAV fan over 15 HP.

The motor and VFD effiency generally decrease with reduction in percentage full load rating for a non-linear curve, meaning empirical data would work best, the fan curves. If you look at a typcial fan curve at two different speeds, note the static pressure in I.W.G., then apply the equation

FLOW * IWG/6,356 = W

you will get a different answer than using the affinity laws. The fan curves not only address change in efficiency, but also the reduced work from operating at lower static.

For a constant flow application without variable loading resistance, first thing I would look at would be reducing pressure for required delivery, just like setting minimum operating pressure for an air system. Re-sheaving or impeller change would make more sense in this case than a VFD.

The ASHRAE HVAC Systems and Equipment Handbook might be helpful on the matter.

 

[ccfowler]

One application that can be well suited to benefit from the use of a VSD/VFD/ASD (everyone can pick the terminology that they happen to prefer) is powering the induced draft and forced draft fans of a large steam generator that must operate a significant portion of its time at much less than full load. This is an application that is nearly pure circulation so when the fans are well sized, they function at near BEP throughout their operating range, and the unit's auxiliary power savings are dramatic compared to other arrangements--such as inlet guide vanes. Not all such applications are suitable. The amount of time spent how far below full load is very important in determining the potential benefits, and it is very important that the fans be properly sized. If fan sizing is off, then the VSD assures that the fan will spend its entire service life well away from BEP performance with energy being wasted needlessly under all conditions.

Based on my observations, nearly all VSD's applied to pumps represent a thorough waste of money and energy. They seem to be applied mainly as a substitute for sound engineering in the sizing of the pump and in the design of the associated piping and equipment systems. The associated attitude seems to amount to, "Why bother with all of those time consuming calculations and when you can just plug in any old pump and VSD and it should work just fine since we won't be wasting any energy through a control valve." I consider finding a VSD attached to a pump to amount to an alarm signal to be wary that this is likely to be a poorly chosen pump connected to a poorly designed system. Shortcuts are seldom taken in only one part of a system or plant.

In general, a VSD driven pump is likely to spend all of its operating life well away from its BEP, and the cumulative waste of energy is almost certain to be completely ignored because of the simplistic (and politically encouraged) presumption of energy savings associated with the mere presence of a VSD.

I agree with the earlier comments about not operating VSD pumps in parallel. Maintaining stable operation for such a system is sure to be found to be in the range from very difficult to impossible.