BFW Pumps Impacts on Boiler Operations 4

[ghamsa]

There are 3 boiler feed water (BFW) pumps feeding 2 water-tube boilers. The BFW pumps supply the boilers with boiler feed water at discharge pressure of 900 psig and with pumping capacity that is good enough to feed the 2 boilers. The 2 boilers provide heating steam for process heaters.
My question is: if the steam demand (from the process heaters) is constant, not changing, and suddenly one BFW pump trips. How this pump trip would impact the boiler operations? For example will there be swell effect?

Many thanks

 

[ash9144]

Not a swell effect at all. Down right bad in fact.

If you lose water you start to lose level in your boiler. Hopefully you have some interlock to cut fuel supply. If not you would eventually over heat the tubes and they would probably fail. Another possibility is areas of the tubes start to heat with no flow and when the pump is restarted the thermal shock ruptures a tube.

 

[BigInch]

Not to mention the possible slams you could get from the hydraulic surge accompanying the pump trip. I would imagine it might be a pretty good one with that 900 psi on top of it. Got some good strong check valves in there?

http://virtualpipeline.spaces.msn.com

 

[MJCronin]

banu....

More information about the system is needed..

Are all three pumps running at te same time, or is one pump an "installed spare" ?

Do all pumps feed a commnon header and are is the system then piped to the boilers ?

Are the pumps ovesized for the service or operated by VFDs ?

-MJC

 

[JimCasey]

I could see a boiler swell happening. If you are running on 3 pumps and lose one, the other 2 pumps run out on their curves so they are producing more flow - maybe even enough- but at lower pressure. The sudden drop in pressure may let the water in the steam drum flash, which is a swell. You will need to install control systems to mitigate the consequences. BigInch suggested a check valve on the steam outlet, although using the capacity of the system to buffer the pressure change might be beneficial. Pressure sensors coupled the the fuel feed would be a lot quicker than waiting for the steam temp to show a change.

 

[BigInch]

No. A check on the pump discharge was what I wanted to say.

http://virtualpipeline.spaces.msn.com

 

[ghamsa]

Are all three pumps running at te same time, or is one pump an "installed spare" ?
For non-routine operation, the 3 pumps will be running at the same time.

Do all pumps feed a commnon header and are is the system then piped to the boilers ?
All pumps have common suction and discharge headers. The system is piped to the boilers.

Are the pumps ovesized for the service or operated by VFDs ?
The pumps are equipped with yarway valves.

Extra information:
Each pump is equipped with a PCV on the outlet.
Each boiler is equipped with 3-element level control system.
Each pump is equipped with check valve on the discharge side
I am not sure, but for the sake of argument, I would assume that each boiler is equipped with interlock to cut fuel supply

Considering the previous replies and the extra information, I can conclude that tripping one BFW pump will have NO impact on the boiler safety. The only impact of BFW pump trip is that the heating steam supply will be limited and can not meet the process demand.

Thank you all

 

[sailoday28]

Casey is correct, note however, that heat is still transferred to the water tubes, further adding to the swell effect.

 

[rmw]

I'm weighing in on this one a little late but I have the following comments.

Swell (or shrink) is not normally a function of the actions of the BFP's but rather the boiler load. If your load is swinging, you will experience swell when the load increases and shrinkage when the load falls off.

About all the water side has to do with this is that the sudden inrush of colder BFW upon a load swing can have a quenching effect as it enters the drum but this is minor compared to load swings and not normally noticed.

If your system is designed properly, the BFP's will discharge at a pressure that always exceeds boiler operating pressure plus rated pressure drop across each boiler's drum level control valve.

With 3 pumps running, all the pumps will be running back on their curves, the pressure will be higher and the FW level control valves will be taking a larger pressure drop. When one pump drops off, the other two pumps will run farther out on their curves, but it is assumed that they will still produce enough pressure to push water across the FW level control valve(s) into the boiler(s).

The 3 element control system is looking at steam flow, water flow and drum level. With the valves at a (pinched back) position where they are taking a larger pressure drop due to the higher pressure of the 3 pump operation, there will be a momentary lag while the controls answer up and open the valves to accommodate for the lower pressure drop due to two pump operation. The steam measurement element won't see any change, but the level will begin to fall slightly until the valves change and the level controller will react to that as will the water flow element input to the control system.

So, in theory you might see a small momentary drop in the drum level, but it is not shrinkage. It is just a level drop due to a temporary reduction in sufficient water flow. The control system will adjust depending on how it is tuned and move the valve(s) accordingly and within a short period, the drum level will be back to normal.

All this assumes a steady load (no effects of swinging loads from the process heaters) and that the BFP's are properly designed and can produce the pressures required for 2 pump operation.

rmw

 

[MJCronin]

rmw,

Thank you for that informative and well written post....!!

Is there a web reference that you can give us for more information ?

Regards

-MJC

 

[rmw]

MJC,

I can't think of any but I will keep trying. Most of the above came from (painful) experience. Ever try to convince an operator who does not have much technical training that he needs to open the water valve to the boiler when the swell has the water almost out of the top of the sight glass? Or that if he knows he is about to be hit by a big load increase that he should lower his drum level a small amount (to prevent priming)? It is counterintuitive. A well tuned 3 element drum level control system (or a good 2 element for that matter) is smarter than your average operator.

rmw

 

[rmw]

MJC,

I found these links via the magic of google.

http://www.federalcorp.com/tech/drum level.PDF

http://www2.sea.siemens.com/NR/rdonlyres/68A2D946-264C-44BD-B1BF-48F831804D71/0/AD353105r2.pdf

The first one deals with the issues discussed in simple easy to understand terms, but the second one gives some better solutions IMHO.

I don't completely agree with the author of the Federal Corp article in his last paragraph of his 2 element control section in that with the use of a differential pressure control valve ahead of the FW regulator valve a constant DP can be maintained across the FW regulator making its flow linear across its range (even and especially if the characteristic of the trim isn't linear). In other words, if the valve is sized for a 25 PSI dp across it, a DP Regulator will maintain dp across the valve regardless of where the FW pumps are on their curve(s).

A 3 element doesn't care in that it measures true water flow and balances that against steam out trimmed by level of course.

The Siemens article correctly shows pressure compensated steam flow meters which are mandatory for boiler control of boilers with any type of swinging loads which most industral/process boilers are to some extent.

Now, after reading the first article and doing some contemplating, let's return to the OP's question and see what happens when his third pump trips. Assume for the purpose of this harangue that the boiler is steady state unless noted otherwise.

With the FW regulator valves set (meaning pinched back) to control drum level with all three pumps running which would mean that the 3 pumps would each be running farther back on their curves than two would, when one pump trips, the other two immediately run out on their curves reducing the inlet pressure to the FW regulator, and hence reducing the total flow of water passed by the valves at that position. So now somewhat less water is going to the drum than immediately before the pump tripped.

As I noted earlier, there will be a momentary dip in drum level until the drum level controller sees the drop in level and its error function begins to call for a change in valve position. How fast this happens depends on its tuning.

This will cause the drum level to drop (at least enough to create the error signal) and continue to drop until the valves open and the increased flow restores the drum to its normal level.

But, too, since and while the inlet BFW flow is reduced, the reduced inflow of cooler (or cold) BFW will cause the firing rate set at that time to increase the pressure in the drum (it has less cold water to heat to generate the steam) so the pressure will rise, which naturally causes shrinkage in the drum level.

At this point, no additional cold water has hit the drum yet, only a reduced flow of what water was being admitted.

The drum level controller will see this shrinkage and will create a greater error signal calling for the FW regulators to open even more, and slug the drum with what the Federal Corp author calls "phasing" which is the sudden introduction of an abnormal amount of colder water which will quench boiling in the down comer tubes and will cause its own shrinkage.

The slug of colder water will also drop the pressure, but this will not cause as much of the swell effect that the author mentions because the water in the drum in the immediate vicinity of the water distribution header in the drum will be sub-cooled for the operating pressure of the drum. The firing rate controller will see this drop in pressure and increase the firing rate to bring all this new cold water back to boiling temperature for the pressure set point, and soon all will be well. (Unless, of course, the process becomes unhappy with the slight drop in steam pressure mentioned above and calls for more steam which would then of course cause swell in the drum-bad if that occurs just about the time that the firing rate increase takes effect.)

So, as I see it Ghamsa, you shouldn't be worried as much about swell in this situation, rather shrinkage and maybe some serious shrinkage. Where are your low water cut off's set compared to drum level?

rmw

 

[ghamsa]

Normal working water level is 55%

alarm water level is set at 48%

low water cut off set at 42%

 

[rmw]

Ghamsa,

That seems tight to me, BUT that said, without knowing your boiler drum size and geometry nor your load swing history, I am not about to recommend a change. If during normal operations you aren't bumping your low water alarm (or high water alarm) very often I'd say it isn't broke so don't fix it.

Everything said before is theoretical but one would have to have intimate knowledge of your entire steam system to offer a valid opinion on your situation.

I would think that the situation that you described in the OP was also rare, so even with these tight settings and the system that you describe (constant load from the process heaters) you will ride through it OK. However, I would discuss it with your controls expert and ask him/her to check the system for such a potential upset caused by the pump trip.

rmw