High Pressure Boiler Feed Pumps 3

[megawattman]

Is anyone aware of why a multi stage boiler feed pump (providing water to a drum boiler) operating at 350 degree suction temperature, 140 PSI suction pressure, 3600 PSI discharge pressure at 5200 RPM would not have a low suction pressure trip or a low supply tank level trip?

 

[pmover]

better safe than sorry!

pay me now or pay me later!

pumps in critical service should have alarms and safety shutdowns to prevent catastrophic failure of equipment/systems.

low pressure trip/shutdown - yes; as the liquid (boiler feedwater) will flash (evaporate) to steam upon a reduction in pressure.

low drum level, the objective is to avoid a potential equipment failure and/or advise the operator that an abnormal condition exists. a pump not pumping a fluid at those conditions would not be prudent.
-pmover

 

[vanstoja]

Is it possible that the 140 psi suction pressure is provided by a main feed booster pump which has a loss of flow or loss of discharge pressure trip control that also shuts down the main feed pump? Your feed pump would seem to need at least 14 stages to keep the stage headrise below the approximate 500 ft/stage beyond which cavitation damage becomes particularly destructive. How many stages does it have?

 

[megawattman]

pmmover
I know why you should have protection, my question is Does anyone have any reasons to design a system without protection?
Thanks

 

[megawattman]

vanstoja
The suction supply is from an pressurized elevated tank (deairator)which does not have any trips for the feed pump. The number of pump stages varies by manufacture.
Thanks

 

[MJCronin]

megawattman,

If the sole source of supply for the pump is the plant's deaerator and if the deaerator has level control/indication and alarm, then the function of the level control is to protect the feedwater pump NPSH

If you get a chance to go back into the history of power plant design, you would find out that the FW pump suction piping is subject to a lot of transients and has been the subject of many ASME piping design papers and discussions.

This suction piping is subject to extreme "fluid inertial" loadings during startup and shutdown as well as during deaerator upset conditions. The physical design of this piping is for minimum pressure drop, minimum retention volume as well as for internal transient pressure surge conditions......In the words of my twelve year old daughter...."its special"

I suspect that it may be much more reliable to control the pump NPSH via the deaerator level than it would be by alarming/recording a pressure that may vary considerably.

My opinion only

MJC

 

[ccfowler]

megawattman,

As in his other responses, MJCronin has shown wisdom and experience that should be heeded.

With the current trends in highly automated instrument and control systems, it is very easy to wind up with needlessly redundant alarm points.

 

[megawattman]

ccfowler
I agree with all the responses, that is not the question.
The question was - Is anyone aware of a reason/design that would not include any feed pump trips?

 

[ccfowler]

megawattman,

Check through your system to see whether there are other alarm and trip points that functionally provide the protections that you think are needed for the BFP.

 

[tonyh]

Of course depending on the system, someone might reasonably be more scared of running his boiler tubes dry for lack of feedwater than getting into pump problem, and therefore be reluctant to trip his pump. A bit of damage to the BFP is more likely to be repairable than a bit of damage to the boiler. Of course you should be able to design the boiler with enough capacity but it is a potential issue.

 

[pmover]

u must b a dxx lawyer and not an engr. u r reminded that this forum site is not for design purposes, but a forum that conducts technical informational exchange btwn users.

while not familiar with your particular situation/design; the short answer is no. the only reason (not a design issue!) i can think of for not including any feed pump trips is a cost issue. which by the way is not a technical issue - in my mind.

you have been provided ample justification for designing trips in bfp systems and sound reasoning by mjc. of course, the choice is yours to take heed.
-pmover

 

[pablo02]

all of the responders have made good points and good info -- in my opinion, there is only one good reason to not include trips on a high pressure boiler feed pump: a system where you WANT the pump to run to self-destruction (I'm not being facetious here, and although I am not familiar with such a need, there may be some applications where this is the desired effect rather than another outcome... a lesser of two evils type situation)