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Boiler Feed Pump Problem 1

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Whyknot

Mechanical
Feb 20, 2007
9
I have a customer who experianced the following which is something he does not want to repeat. Does anyone have an idea what caused this?

The Pump is a 3GT I-R Boiler Feed Pump, Steam Turbine Driven, 240-257 deg F water from DA tank
Pump was shut down and not turning but hot or at least warm
Discharge valve downstream of pump was closed first. This valve was some distance away from pump (20-30’?)
Pump has discharge check valve
Bypass Line to DA Tank from Pump discharge was open
Suction Isolation Valve (Gate valve) was manually closed – takes maybe 30 seconds
There is an expansion joint on suction side next to the pump
A loud explosion/noise was heard by the person closing the suction valve immediately when the suction line valve was closed and there was violent movement of the suction line to such an extent that the hold down anchors for the suction line pipe support (next to the pump) were ripped from the floor. The expansion joint was extended to its maximum.

Operator was visibly shaken and doesn’t want to go through this again


If you have any ideas please let me know. I have asked the customer to check on a couple of theories I have but I want tell you what they are as I don’t want to color your thinking.

 
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Sounds like surge, ie. water hammer, if as I am assuming they apparently had flow in progress from the suction through the pump to the deaerator when the suction valve was closed, since that branch to the DA is upstream of the discharge isolation valve. Is it also upstream of the discharge check too, or is the discharge check immediately following the pump discharge nozzle?

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I will have another look at the piping an let you know. Little more information. I am told the the pump was absolutely stopped. I have having the customer check and may sure we did not have any leaks in the seals on the pump or valves. This is one of 6 pumps and at least 3-4 others were running. The piping system is not new as the pumps have been in the same system for years.
 
Just trying to determine if it was (is) it possible for the DA or any other downstream pressure point to flow back towards the pump if the pump is shut down?

Are all pumps discharging into the same header?

Has a check valve been repaired since then?

It may not matter about the check, some are slow enough acting that fast transient surges are transmitted upstream right through them. Saw that one happen on a nuclear plant feedwater sys. They had a relief valve on the suction side to limit suction pressure buildups when the pump shutdown. You might be experiencing something similar.

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Do other pumps tie into the same bypass line to the DA? If so, the bypass flow from these pumps could have split, with some fraction going backward through the pump to the suction line.

Since the expansion joint was stretched out, it would point to flow from the pump to the suction at the time the suction valve was being closed.

 
rzbk,

It does seem like there was a high pressure source from the pump discharge side and, since the pump was stopped it does indicate some vulnurability to reverse rotation and backflow from the discharge side to suction, although pump component failure could also be responsible. The slow closing time of the suction line would point towards no generation of a suction line surge, as would a well placed working check valve in the discharge. It should also be somewhat easier to tear up supports with the higher pressure source. The description of an "explosion" would say that whatever happened occurred in a millisecond or so as well, so the possibility of reverse rotation being the cause lessens. To me, it is shaping up like a pump component failure of some kind. Is that how you see this so far?
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BigInch,

Thought process was the following: (lots of assumptions)

Since the expansion joint was stretched, but not ripped open, I expect your first post was on track with surge or water hammer. Once the initial hammer moved the pipe, it appears that there was no high pressure source or the bellows would be ruptured. This would rule out a leaking discharge block valve. (I am assuming ~15# suction and several hundred# discharge)

The bypass to the DA, I am assuming was some sort of minimum flow line back to the suction source. It was left open.

If other pumps tie in to this bypass line it could create some backflow through the pump. (a twin path would exist back to the DA for the other pump) A significant flow might be created through the pump without being enough to cause reverse rotation.

When the suction valve was closed, it would close the path backward thru the pump, and forced all bypass flow from the other pump back to its main path. With a large gate valve and a relatively small flow, closing might not affect flow until it was pretty much seated (last turn of the handwheel).

Again, this extrapolates quite a bit from the original information.
 
Yes, not much to go on initially, however since this isn't my/our problem per say, I enjoy "pre-speculation". I think it was surge, but not exactly sure how it made through the pump just yet. If its not got much of an inertia load from the driver, as might be the case with a light turbine, perhaps it was reverse rotation. The suspense continues.

 
I don't think your customer is telling you the whole truth or all of the story. Closing the discharge valve will not cause problems since the bypass back to the DA will handle the minimum continuous flow conditions for the pump. but by closing the suction valve this would cause the suction pressure to drop drastically causing the remaining water in the pipe and pump to flash. I have seen this before. The operator may not want to fess up to doing something that he was the cause of. For a system that has been operational for years and years and all of a sudden something this catastrophic to happen, you have to look at operator error when it all boils down. (literally)
 
So you think the pump was not stopped, boiled and blew into the suction. Don't have a lie dector, so can't discount that one.

I agree it was probably operator error of some kind. I will also bet that the other pumps are running right now, which would make be wonder why they would risk it if indeed they thought pump design was at fault.

 
I tend to favor the simplest answer in a problem like this. I suspect that the pump pressured up to discharge pressure as the suction valve was closed and this was the source of the problem. The source of high pressure could have been a leak backward through the discharge block valve and check valve. Or, it could have been pressure from a common spill-back line to the DA if the valve on the end of that line at the DA was blocked in by mistake. But simple leakage back through the block and check would be the most likely.

You stated that the minimum flow spill-back was open to the DA but did not mention how this spill-back was controlled. If there was an automatic spill-back valve (Yarway type), it would have defaulted open with no flow if it was working correctly. But a control valve could have been closed depending on the logic programmed into the control system. An orifice or pinched globe valve should have allowed the excess pressure to bleed off. But then we are back to the possibility of operator error. And once the failure occurred, the operator could have opened the valve that he had accidentally left closed. Human nature would lead some to try and cover up their mistake.

I suspect the following scenario. There was leakage back through the block and check on the discharge. The spill-back line to the DA was blocked in or was otherwise restricted. As the suction valve was closed, pressure built up. The operator didn’t notice that the suction line expansion joint was moving. It was only when the bolts snapped off (explosion) that he turned to see the suction line bellows extended and the pipe supports broken. Depending on the pressure in the discharge and the design of the expansion joint, it may not be surprising that this joint extended but did not rupture.

More importantly than all of this, I need to point out that this system is very poorly designed. If there must be an expansion joint in the suction line, it should be outside of the block valve. The expansion joint should have tie rods in place and pinned with slight clearance so that it cannot extend as you describe. If the expansion joint is in a location that could be exposed to discharge pressure by valve failure or operator error, there should be a PSV to relieve the pressure back to the DA. Operators close the wrong valve all the time. The system needs to be designed so that a simple error in the order of closing valves won’t result in serious injury or equipment damage.

Johnny Pellin
 
fwiw - I vote for Jpellin's scenaio

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Of course the cause for the surge needs to be found and cured but there is a side issue that may also need to be addressed.
Whyknot wrote:
"There is an expansion joint on suction side next to the pump
A loud explosion/noise was heard by the person closing the suction valve immediately when the suction line valve was closed and there was violent movement of the suction line to such an extent that the hold down anchors for the suction line pipe support (next to the pump) were ripped from the floor. The expansion joint was extended to its maximum."

I was taught that as a piping designer we should never use expansion joints or flexible connections in pump piping. I now see that there is support for this.
Mr. Heinz P. Bloch, The Reliability and Equipment Editor for Hydrocarbon Processing has written article (Hydrocarbon Processing, May 2007) which addresses this issue.
I am not going to reproduce his whole article but I would like to share a few quotes taken from his article.

"...aside from the fact that API specifications require 'hard piping' for refinery pumps..."
and
"...because flexible pump connectors are the weakest link in the piping, it would be unthinkable to consider employing these connectors in refineries or petrochemical plants."
and
"Thrust loads are anchor loads that only exist when an expansion joint is used and under pressure. and is equal to the pressure in the system times the effective area of the joint"
and finally
""But even on water pumps flexible connectors can often be traced to sloppy engineering."

I hope that someone will take the time and try to improve the design and eliminate the expansion joints.
 
The fact of the presence of the expansion joint may indicate that the piping design was faulty to start with. PennPiper makes a good point and I concur that expansion joints should be un-necessary especially in this fairly non rigerous service if the piping is designed correctly. I have rarely seen EJ's in BFP from DA suction lines.

So there may have been some stresses or preloads on the pipe anchors that were contributory when the operator did whatever it was that he did.

Get the truth. I have had too many assurances that a situation was just so only to go to the plant and find out that I had been lied to by someone to cover their back side.

rmw
 
This seems to be a variant of the classic bearing failure problem: Burned out oil ring bearings always seem to have their reservoirs full of cool, clean oil of the proper grade.

Somebody (most likely more than one somebody) messed up and straight stories will remain in short supply for a long time.
 
Comments from my wife a piper with 35 years experience.

It appears that we do not have all the info. What size are the suct & disch lines and what is the op press? Also, my first instinct was that the weakest point failed. I am guessing on this but- was the discharge valve closed first with the pump still pressurized? If so then the pump may have dead headed which would leave the suct side to pressurize or perhaps over press and the weakest link would have failed. This could have been gasket, valve seat, ex joint, etc. Without having all the info I would guess at this.
 
there are some holes here that i don't like as well. do you have more information?

what is the suction and dischage pressure typically? is this a 40 psig boiler or a 900 psig boiler?

what is the MAWP of the pump casing, its isolation valves and the expansion joint?

how do we know the pump was stopped? since this is steam driven, when was the steam shut down?

although i also don't like the expansion joint failure, if it was due to the suction valve being closed first and then it failed to the extent it did, why did it stop failing? why didn't it just fail completely and blow out? how often do you see a reverse buckling rupture disc pop backwards and not burst? not a great analogy but you sort of get the idea.

because of this, i would lean towards water hammer that caused sudden damage but then stopped.

a spinning turbine and a blocked discharge valve followed by a closed suction valve could cause rapid cavitation and hammering in the suction. i have seen the pounding in piping from water pumps being blocked in when a centrifugal pumps started. it was a frightening noise that ensued.

and i agree whole-heartedly with the PSV on the suction of the pump if this is a chance for a procedural mistake but what size do you need for the full line size to be open, this does not seem feasible.

anyways, just hoping to add some ticklers to stimulate any thoughts and discussions you are having there.

good luck and keep us posted with any findings!
 
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