Parallel pumps operation 1

[geliberman]

I've recently integrated a new pump at a boiler feed water plant that is in addition to three existing pumps.
Three old pumps are identical(2 motor driven and one steam turbine driven pumps) with 73m shut off head.
The new pump (Steam turbine driven) is smaller and have 58m shut off head.
When we use 2 steam driven pumps,(new and old) there is no problem, but when the third pump starts it will cause the new pump to work without flow and the pump check valve will fluctuate,and after a while the casing temperature will increase. the cavitation also occurs in the new pump.
I know that we have made a mistake in pump selection, but how can we solve the problem.
How we can change the system curve to use 2 old pump together with new pump?
is that possible to use ARC valve or it is better to use a control valve?

 

[bimr]

It is difficult to say without a more complete understanding of your system. It should be obvious that the newer pump does not have enough discharge pressure to pump in parallel with the existing pumps.

Presumably the existing pumps were installed with a discharge head that is appropriate for the operation of the boiler. If that true, then you probably need a pump that has a higher discharge head. Perhaps a larger impeller may be installed on the newly installed pump to increase the discharge head.

 

[JJPellin]

I would take the curves for the new pump and the old pump and overlay them on the same chart. Since the new pump is driven by a stream turbine, you may be able to speed the turbine up to get higher head from the new pump that is a better match to the curves of the old pumps. Don't focus on the pump shut-off head. You need pumps that can run together at their normal operating point. If the shapes of the two curves are different, you may still have one pump that has a higher head at shut-off, but a good match at normal operating flow. You can adjust the curve of the new pump for possible the possible speed increase by using the affinity laws.

As a note of caution, any speed increase on the turbine needs to be carefully analyzed. You need to verify that you will not create a hazard related to the maximum continuous speed of the pump or turbine. You need to verify that you will not create a potential for pressure higher than the downstream system can accommodate. You might also need to adjust the overspeed trip set-point of the turbine. Consult with the turbine manufacturer for all of the possible implications of the speed increase.

Another alternative could be to install an orifice in the discharge lines of any pumps that have a flatter curve to make them behave as if they have a steeper curve. A spreadsheet program could be used to adjust the turbine speed and orifice size to get the best match over the entire operating range that is required.


Johnny Pellin

 

[geliberman]

Dear Bimr,
Thank you for your early reply,
please find herewith enclosed the pump Curves

 

[stanier]

Check out for parallel pump operation with different curves.

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http://www.pumpsystemsmatters.org

http://www.pumpfundamentals.com/pump_glossary.htm#...

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[Artisi]

Without more detail it is hard to give a definite answer, but it would appear that even though in theory the new pump and the old pumps appear to be compatable ie, rated at round 10.8 M3/hr at 54 metre head there is something else going on we don't understand at the moment.
I would suggest that the 2 old pumps are running at less than 10.8 m3/hr (each unit) but at a higher head than 54m (probably in excess of 58m) - therefore the new pump is not capable of running against the actual operating head.

You need to establish the flow rate and discharge head on the system with the old pumps operating and then look at the likely changes of flow and head with the 3 pumps in operation.

Bear in mind that putting a 3rd pump into the system ( I guess you are trying to increase the flow rate of the system) you will further increase the discharge head - more friction losses - forcing the old pumps further left on their curve which increases the discharge head.



It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[JJPellin]

Judging by the curves, it looks like a good match at rated flow, but a poor match at lower flow. The shapes of the curves are the problem. Even if the pressure and flow are identical at rated conditions, the fact that the new pump has a much flatter curve creates a problem. As you add pumps to the group, the flow from each individual pump is reduced. The old pumps move left on the curve and produce higher head. The new pump slides all the way back to shut-off since the flatter curve of this pump cannot match the head of the old pumps at the lower flow point. As I noted, you should be able to speed up the turbine to get a better match at the lower flow. Overlay the curves and then play with the turbine speed to see how close you can get.

If you added an orifice to the new pump, you could probably get a very close match across the entire curve. The orifice plate will make the curve steeper. The speed increase will increase the head at shut-off. In combination, you can achieve the shape you need.

Johnny Pellin

 

[bimr]

You need to go back to the pump supplier and your equipment specifier to understand how this situation developed. You should provide the pump supplier with your maximum flow requirements which is the reason for operating multiple pumps.

You have a choice between increasing the speed of the turbine as JJPellin suggested or a larger impeller if that is possible. Either of these proposals will work to increase the pump discharge pressure. The pump supplier should be able to recommend the best alternative.

Neither of these solutions will probably be an ideal solution since the curves do not appear to match. It may not be possible to obtain the maximum flow that you desire which is the reason for the multiple pump operation.

In general, steeper pump curves are recommended for parallel pump operation.

I am not sure that there is much benefit to adding an orifice plate. Adding an orifice will work to shift the pump operating point leftward. Even if you add an orifice plate, the pump discharge pressure still seems to be inadequate to compete with the other pump.

 

[TenPenny]

I would be willing to bet that the flow rate is lower, and therefore head is higher, than expected, or, alternatively, speed and/or impeller diameter of the old pumps are higher than thought.

From your description, the new pump is not able to overcome the head of the old pumps.

I find that it's most helpful to have actual pressure readings of the discharge of each pump, instead of relying on where it was selected to run based on plant design.

 

[CaracasEC]

According to the situation, running the old pump driven by steam turbine with the new pump driven by steam turbine pump parallel operation is not a problem.Adding the third pump driven by the motor gives problem to the new pump this is because each pump flow rate will decrease moving the operation point to the left of the curve on which we are dealing with a new pump with a flat curve and old pumps with step curve. Recirculation or backflow is being experience by the new pump. For this situation what i think is that i agree that an impeller change is a better option with the expertise of the pump vendor.

 

[Artisi]

Lets establish the existing flow and head before jumping to conclusions that may well be way off.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[1gibson]

Here are the problems:

1) You requested a 4th pump with the same design point as the existing three pumps.
a) We all know if you plan to run all 4 at the same time, flow will be reduced
b) This may have made sense at the time if it was to be an "installed spare" to still allow two pump operation if one of the originals is down for maintenance.

2) The selection wasn't reviewed thoroughly for parallel operation
a) Pump vendor did not have enough info, or care enough to review it, or was led to believe that it would actually operate exactly at the "rated point" so deemed the selection acceptable
b) Purchaser did not review thoroughly enough. I suspect it was a case of whipping out the checkbook due to an urgent need for equipment.
c) Both parties could have helped avoid the situation, but keep in mind that all a salesman needs to sell a pump is a design point and a customer's purchase order, the rest is just silly little details.
d) NEVER trust an OEM equipment salesman with an aftermarket/retrofit/ "like for like" replacement job unless you are replacing all of the equipment. If the new stuff has to work with the old stuff, find a dedicated aftermarket sales engineer.

3) The logic for adding another pump may have been flawed, too nebulous to explore without some additional information, but a re-rate at next maintenance opportunity (buy larger impellers and have then on-hand) may have been best.

 

[Artisi]

1gibson,

I think your point d) is a bit over the top, you are assuming that all OEM salemen don't know what they are doing. I would suggest that there are many OEM "salesmen" who would run rings around some of the so-called "aftermarket sales engineers".

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[1gibson]

No offense intended, but hearing "yes everything checks out fine" from an OEM guy is different than hearing it from an aftermarket guy. The expectations for detailed review of a project are just vastly different between OEM and aftermarket. I mean "trust" in the literal sense, to take them for their word without any evidence of equipment compatibility. Not that I would trust either without seeing multi-pump curves, but of course I am always skeptical. Not a luxury everyone can afford, eventually you have to trust someone.

Interesting that you comment on 2d but not 2c, if you were to take offense to either, 2c is much more flagrant (and I will admit, maybe a bit over the top.)

 

[Pumpsonly]

Obviously the OP's intention is to run 3 pumps in parallel to get more flow without realizing that 1+1+1 is not equal to 3 in this situation. With increased flow with 3 pump running, the total friction losses also increase,there by moving the operating point of each pump to the left even if all pumps are identical.
First you have to find out what is the total flow you need for running 3 pumps and recalculate the new total head required. Checked the existing pup curve if it still can meet the new head required.
Specified the new pump flow rate by 1/3 of the total flow and the increased required head with shut off head more less the same as the existing pumps if there is no changes.

There are possible way to make the new pump works with the old pumps in your current situation.

1) Increase the turbine speed or install a bigger diameter impeller to get the same shut of head as the old pumps.
2) Use the affinity law to plot the new curve.
3) Installed an orifice plate at the discharge flange to reduce the discharge pressure /diff.head at rated flow to 54 M
.

 

[TenPenny]

Without knowing what the piping system is, even the comments about 1+1+1 not being equal to three are meaningless.

The whole lot is speculation based on not enough information to draw any conclusion.

 

[bimr]

Most people with 20 years in the business would know that majority (75+%) of the boiler feed pump discharge pressure is pressure necessary to overcome the boiler pressure. The pressure loss through boiler feed piping is generally relatively small because of the short length of pipe, maybe 10 psi. The control valve pressure loss is probably within the range of 10-20 psi.

You would also know that it is a high pressure boiler because he is pumping DI water.

 

[Pumpsonly]

Where did the OP said is pumping DI water or is a boiler feed water pump? He only mentioned is "boiler feed water plant".Is 54M head considered high pressure? If you try to push 50% more water through the pipe , the friction head increased by the factor of approx. 2.25 times. From 10 PSI losses at 2 x 10.8m3/min to 22.5 PSI at 3 x 10.8 M3/min.

 

[Artisi]

Must make a small correct my comments, I mistakingly thought there were 2 existing pumps but on review I see there are 3 PLUS the new unit, not that it changes anything - just replace 2 with 3 and 3 with 4.

We are still working without a full understanding of the problem and until such times the OP enlightens us with more detail it is still all guess work.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[SeanB]

To Johnny Pellin...
"If you added an orifice to the new pump, you could probably get a very close match across the entire curve. The orifice plate will make the curve steeper."

I have heard this statement before from someone but I have never understood what is actually means. Add an orifice where...in the discharge? a minimum flow spillback? Can you explain to me what this means?

Thanks.