NPSHa Margin for Condensate Pump
NPSHa Margin for Condensate Pump
(OP)
Is there any recommendation on NPSH available margin over NPSH required for condensate pump in power plant? Some designer give no margin, what's going to happen when the plant is in operation?





RE: NPSHa Margin for Condensate Pump
If you don't have a "comfortable" margin between the NPSHa and the NPSHr when you're trying to pump a saturated liquid, you're going to have pumping problems: the pump will loose prime and fail to pump. Since you don't tell us, I don't know why your designer would give no margin except that it is foolish and dumb not to do so.
I've never had my pump designs fail due to NPSHa problems and I've pumped a lot of saturated liquids in 45 years - from the cryogenic liquids to steam condensate. My designs always stipulate:
NPSHa > NPSHr + 5 ft fluid
or,
NPSHa > NPSHr * 1.35
whichever is larger.
There's a big difference between gasification or loosing prime and "cavitation". "Cavitation" if frequently a misnomer applied to gasification and loss of prime.
RE: NPSHa Margin for Condensate Pump
The reason may be that the selected pump is intended to operate in the break, aka on "submergence" control.
Such a pump must be carefully designed (stage pressures and rpm) and is supposed to cavitate at all flow rates. However, this cavitation is not considered damaging because the shock waves created by bubble collapsing don't have enough intensity to become harmful.
I recommend you read the subject of condensate pump regulation of Chapter 9.5: "Steam Power Plants" in the Pump Habdbook by Karassik et al., McGraw-Hill.
RE: NPSHa Margin for Condensate Pump
On more than a few occasions, when phisical limitations prevented doing what Art suggested (for example; deaerator could not be raised to increase NPSHa) increasing the suction pipe size to get the velocity below 6 fps would give the desired result.
rmw
RE: NPSHa Margin for Condensate Pump
RE: NPSHa Margin for Condensate Pump
Is there any way you can flow limit the pump so that it does not exceed its NPSHr margin?
rmw
RE: NPSHa Margin for Condensate Pump
RE: NPSHa Margin for Condensate Pump
What we found out was that most manufacturers pump curves understated the NPSHr curve. That is to say if they said it needed 6 feet, it usually needed about 7.
We knew whose curves were accurate, and whose weren't. Unfortunately, the ones that weren't were from some of the biggest names in the business.
Some of the products we made went on military submarines and surface vessels. Finding out that the pump curve lied during sea trials was not cool.
Be suspicious of pump curves until you know. Part of the allowance that the responders to your thread are recommending is for that proviso also.
rmw
RE: NPSHa Margin for Condensate Pump
I am a bit late on this one - however maybe I can help a bit :
When due to the Projects constraints there is an unsatisfactory difference between NPSHA and NPSHR ( and this issue was extensively tackled by previous postings)there are few other things that might be considered :
1. Working to minimise NPSHR :
a. Consider fitting the pump with an Inducer.
b. Ask for pump special manufacturing attention - by controlling during the manufacturing process few aspects such as quality of liquid exposed surfaces ( both for casing and impeller ), underfilling of the impeller , cleaning eye of impeller, etc, the actual NPSHR figure can be usually improved. Any reputable pump manufacturer has some tricks in his sleeve for special applications.
2. Always ask for a NPSHR Test before taking delivery of the pump. This will protect you from underperforming pumps.