Pump start and time to begin flow

[gmax137]

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.

 

[JJPellin]

This is not a good situation for the pump. I assume you are talking about a centrifugal pump. Without more information about your system, it is difficult to make an assessment. But a few thoughts come to mind.

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

 

[vpl]

gmax

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

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[BigInch]

Do you have static pressure to overcome before flow begins in the pump discharge line. If there's a discharge check valve you can tell, because the check will remain closed until static pressure is reached and flow through the pump begins. Whether the check is there or not is immeterial, except to prevent possible backflow from discharge to suction when a high initial static pressure is present. There could also be a delay if the discharge line is long and worse when its uphill. All that water mass in the piping has to be accelerated. In long pipelines 100 km or so, it can take 5 to 10 minutes to ramp up the flows. For that reason it is often necessary to wait a few minutes while suction pressures build up at, and before starting, the next pump station.

**********************
"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/

 

[gmax137]

Thanks you all for your replies - you have given me a number of things to think about. I am looking for more historical data, and I am looking into the details of how this data is actually recorded. I still have doubts as to whether the 'clock time' on the pump speed is the same as that on the flow rate.

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

 

[BigInch]

Actually those specific booster stations were diesel (no electricity in the area), so they had to be started and warmed up for about 10 minutes before being able to deliver full power. They would idle during that time and run the pumps with the recirculation lines open. Product discharge temperature was monitorerd. Discharge was spilled back through a control valve and orifice plate to the beginning of a long large diameter suction line. The thermal mixing occuring in that suction line was sufficient for a 20 minute warm up period. That warmup period was about twice what was needed to line up the product tank, valve manifolds, start the electric driven mainline shipper pumps to begin line packing to bring suction pressure up at booster #1. If everything went right, it hit 350 psi, start #1's boosters and draw suction down pretty fast to 75 psig. Being that the first station was electric, it usually went right on time. Booster station #2 would at the same time as #1 and do the same thing. It was only 20 km between booster 1 & 2, so that segment didn't take much packing time. It usually also had 1500 meters of static head holding a pretty tight pack anyway. #2 also lifted almost the same 1500 m, so pumps at both stations took almost full power just to open the check valves before any flow would occur into the pipeline, which is why the recirculation line was needed. Most installations don't have such a large static head to open the checks, so you can flow packing volumes into the discharge pipeline segments and keep the pumps cool without recirculation lines.

**********************
"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/

 

[vpl]

gmax

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 for tips on how to make the best use of the Eng-Tips Forums.

 

[gmax137]

Well it turns out that the data on pump speed and the data from the flow elements is recorded on two separate computers, and the clocks are not synchronized. So plotting the two data sets on one plot produces a mysterious and misleading graphic. The 20 to 30 second offset in pump speed relative to flow is not real.

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.