Pump start and time to begin flow
Pump start and time to begin flow
(OP)
I'm looking at some recorded data that shows both pump rpm and flow (measured a ways downstream). There seems to be a long delay between the time the pump is at speed and the time when flow begins. This is for a steam-turbine driven pump, so I expected to see a delay from when the steam supply to the turbine is opened to when the pump is operating. But I see the turbine coming to full speed well before flow develops (say about ten seconds). I'm not sure whether to believe the data or not (maybe the time stamps are not synchronized, or some other problem). Do pumps really behave like this (spin for awhile before the flow gets moving)? Does it depend on the length of the discharge piping? The pump is making about 1200 gpm, pumping cold water. Thanks.





RE: Pump start and time to begin flow
Suction Problems - If the pump is trying to achieve suction lift from a sump below the pump elevation, it could have an air leak that results in an air bubble that has to be displaced before full performance is achieved. This could be very damaging to the pump. Bushings and wear rings in the pump are designed to be full of liquid. If they are passing vapor while the pump regains prime, there could be excessive shaft deflection, rubs, high thrust loads, etc.
Discharge Problems - If the discharge line is not liquid packed at start-up, there will be a delay while the pump floods the line up the flow meter. A typical flow meter in our plant is an orifice plate with a DP measurement that correlates to flow. With gas passing through the orifice plate, there won't be enough pressure drop to register any flow. In this case, the pump is actually pumping at a very high flow rate until the line fills. This can also damage the pump. Running at higher flow brings along a higher Net Positive Suction Head required. The pump can cavitate during this period. There can also be higher radial and thrust loads depending on the configuration of the pump. In a vertical turbine pump, the shaft could buckle. In a horizontal multistage pump with a balance piston for thrust compensation, the thrust bearing could be overloaded and fail.
If you could describe the configuration of the pump and the system you would get more useful replies.
Johnny Pellin
RE: Pump start and time to begin flow
I used to have some charts that showed the time span between when a particular steam turbine driven pump received the start signal, the stop valve opened, the turbine came up to speed, and flow actually started. Unfortunately, that project got resolved, so I no longer have access to those charts.
However, from what I remember, there was a fairly considerable (on the order of seconds) delay between when the turbine came up to speed and the water started flowing. I want to say the time frame was around 23 seconds, but that's a pure swag on my part and may only have been applicable to the particular pump.
Patricia Lougheed
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RE: Pump start and time to begin flow
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"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Pump start and time to begin flow
BigInch - my discharge pipe length is in the hundreds of feet vs. your kilometers of pipeline, but your remark makes me wonder about the effects on your pumps if they take 5 or 10 minutes to develop full flow. What happens during this time? Do the pumps turn at rated speed without delivering full flow (kind of 'spinning their wheels')? Or do they slowly wind up to rated speed?
Thanks again
RE: Pump start and time to begin flow
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Pump start and time to begin flow
Since your handle says Nuclear, if you're at a nuclear plant (at least in the US), there are probably strip charts from past startups and tests being stored in Document Control.
Even if you're offsite, if you're working on a plant issue, you should be able to get copies (though the readability of those can be very iffy.)
Patricia Lougheed
Please see FAQ731-376: Eng-Tips.com Forum Policies: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.
RE: Pump start and time to begin flow
Patricia - if you have occasion to view data from a "PI" system be wary: in addition to this clock issue, there are further subtleties in the way this system collects data. Best to understand this before drawing any conclusions from the data.
Thanks again to all who responded on this question.