Pump Coupling - Pump Deadheaded
Pump Coupling - Pump Deadheaded
(OP)
I'm not particularily familiar with "large" pumps, so forgive me if this is a ridiculous question.
We have a boiler feedwater pump for a 500bhp boiler. We were having some issues with a valve and flow transmitter on the inlet of the boiler, so we were runnign the system and dead headed the pump a few times.
My question is, does this cause excessive stress on the pump coupling to the motor?
The reason I ask is that we recently noticed that coupling has seen excessive wear and had to be replaced. Could this have been caused by us closing that valve?
Any input is appreciated. I will check back to provide any other information that could be helpful.
We have a boiler feedwater pump for a 500bhp boiler. We were having some issues with a valve and flow transmitter on the inlet of the boiler, so we were runnign the system and dead headed the pump a few times.
My question is, does this cause excessive stress on the pump coupling to the motor?
The reason I ask is that we recently noticed that coupling has seen excessive wear and had to be replaced. Could this have been caused by us closing that valve?
Any input is appreciated. I will check back to provide any other information that could be helpful.





RE: Pump Coupling - Pump Deadheaded
Good on ya,
Goober Dave
RE: Pump Coupling - Pump Deadheaded
When you shut off the discharge flow, the water that is inside the impeller housing doesn't just stop. The impeller is still acting on it, so water is still moving from the impeller ID to the OD, but it can't go on around the volute and out the discharge, so it flows down the sides and re-enters the impeller ID. This changes the forces that the impeller, and thus its shaft, experience. You're putting that HP into a fairly small, closed system. Different impeller designs handle these forces differently; some accomodate it, some do not.
RE: Pump Coupling - Pump Deadheaded
I'm not sure if we ran it "long enough". How long would you say is long enough to see cavitation?
It is a centrifugal pump. Direct drive, 30HP.
RE: Pump Coupling - Pump Deadheaded
tr1ntx's reply makes a lot of sense too. Could be a geometry thing.
How long to boil the water? If your 30 HP pump is, say, 70% efficient, that means about 30% of the input power goes to heating the water -- not a problem as long as the water is once-through. Closing the discharge valve confines a small volume of water in there, plus runs the efficiency down very low. Together, you can get boiling.
Input power is much lower at dead-head, but so is efficiency. Even if it's only drawing 5 HP from the motor, you might have half of that (or more) going to water heating, the rest goes to moving the water from inlet to outlet and back again. Half of 5 HP is 1.9 kW, and if you only have about a gallon of water in the pump housing it will rise almost 13°F per minute. Heat loss through the housing (if uninsulated) will help limit the rise and extend the time, but if well insulated, it might take less than 10 minutes to get to boiling temperature.
I made a bunch of assumptions in the paragraph above, maybe one of the more experienced pump people can correct anything I stated that might be out of line. I have a little practical experience, having boiled a pump once a long time ago. My pump didn't hurt its coupling, but it did blow steam out the seals. It was a 200 HP pump, high speed impeller that was small and thus not much water volume in the housing. I didn't time how long it ran deadhead, but it wasn't too awful long, maybe 5 minutes?
Let's see who else pitches in with better knowledge and experience...
Good on ya,
Goober Dave
RE: Pump Coupling - Pump Deadheaded
In regards to deadhead time, it probably would have been deadheaded for 5-10 minutes at the absolute max, but it would have happened a few different times during the day.
Thanks again for the responses, I just wanted to know for a warranty claim if it could be our fault, and sounds like it's possible. It's not expensive, but something to avoid we'll try to avoid doing in the future. Looks like we'll suck up the cost of this one.
RE: Pump Coupling - Pump Deadheaded
1 Pipe stress. Disconnect the suction and discharge pipes and see if they spring out of alignment.
2 Check alignment figures. Have you checked for soft foot.
3 Sounds like you have a rubber coupling, if the problem persists then check concentricities of hubs to ensure everything is running true to shaft centre line.
RE: Pump Coupling - Pump Deadheaded
Yes, the energy usage of the pump will be increased by this by-pass flow, but you will find it to be a trivial burden compared to the burden of pump system damage. Also, since the problem has been severe enough to damage the shaft coupling, it is likely that there will be damage to the pump internals as well. It is likely that pump performance and efficiency have suffered from this type of operation and the resulting damages.
Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.
RE: Pump Coupling - Pump Deadheaded
RE: Pump Coupling - Pump Deadheaded
Although the pump heating during blocked flow operation is surely less severe here than in the BFP's of my experience base, it is also likely that this pump has been operated in this mode far longer than may really be prudent to avoid internal damage regardless of the coupling damage. I've seen plenty of smaller pumps turned into mechanical water heaters due to inadequate flow (usually by virtue of unfortunately mis-matched pumps operating in parallel). These pumps never operated long in this mode without suffering serious internal damage (what a surprise!).
Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.
RE: Pump Coupling - Pump Deadheaded
If there is axial force on the pump, it's range of motion will usually be restricted by the pump thrust bearing, but that doesn't rule out fatigue stresses to the coupling as I think suggested above.
We do have one set of single stage centrifugal pumps driven by sleeve bearing motor through shim pack coupling where we see large 0.125" axial movement of the motor shaft at a frequency of maybe 1 hz which occurs only when the pump is at low flow (goes away at higher flow). I figure that the broadband / imapcting forces from small movements of the pump rotor (<0.020" pump bearing clearance) act as base excitation for the spring / mass system consisting of the coupling / motor, and excite the resonant frequenc of that system which willbe quite low due to flexible shim pack coupling and large motor mass. That low-flow operation recurs periodically for periods of hours to days, but we have not seen cracking of the shim packs.
You might want to tell us what type coupling and describe the bearing configurations on pump and motor.
=====================================
(2B)+(2B)' ?
RE: Pump Coupling - Pump Deadheaded
Vibration analysis can tell you the story of what's going before the next failure occurs. As others have indicated it sounds like an alignment issue.
RE: Pump Coupling - Pump Deadheaded
Also an off chance of higher pressures affecting the piping and nozzle loads, causing movement of the pump relative to baseplate which would of course affect alignment. Grouted baseplate, or I beam / C channel?
RE: Pump Coupling - Pump Deadheaded
RE: Pump Coupling - Pump Deadheaded
I don't see where anyone mentioned deadheading intentionally.
RE: Pump Coupling - Pump Deadheaded
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.)