having difficulty in understanding the requirement for start at end curve requirement

[Ahmed Khan]

hello, i have newly joined O&G industry and i have a difficulty i understanding the following statement:
"Pump is in Auto-start service. Motor shall have torque capability to accelerate the pump to operating speed with
discharge shutoff valve open, and the discharge check valve closed against minimal backpressure (start at end of curve)."
Can someone please guide through the statement and what it does exactly means.

 

[LittleInch]

The pump will start from zero to full speed in about 3 to 5 seconds.

What this says is that at start there is very low back pressure so the pump will go to the end of its curve with high flow and low head compared to "normal" flow. This is max power as efficiency is low. Max power at the motor fixed speed means max torque required.

Does that make sense?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Ahmed Khan]

Yes, thanks but can you elaborate more regarding the point of low back pressure pls?

 

[Ahmed Khan]

And what will happen of the discharge valve shutoff is fully closed?

 

[shvet]

And what will happen of the discharge valve shutoff is fully closed?

The starting current will be much lower and electric system suppluing this consumer will be cheaper as it won't require additional capacity to withstand intermittent staring overload.

 

[LittleInch]

The low back pressure is stated in the original OP.

This happens when the majority of the head/ pressure produced by the pump when flowing in normal operation comes from frictional losses in the downstream system as opposed to static pressure or head. So if not controlled, when the pump stops, the pressure decays to some low level. When the pump restarts it takes some time for flow to establish the same back pressure and therefore at pu,p stat, the pump goes to the end of the ccrve.

If the discharge valve is closed, as soon as the oump starts to move the head climbs at the same speed and neds less torque to accelarate.

Normally start current is based on multiples of FLA. However here is still benefit from nomrally startjng against a closed valve.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Starting closed the pump is trying to get to operating speed and building pressure while lifting only a minimum internal recirculating flow, so rpm increase is faster and power load during start is much less than when a valve is open and the pump is trying to simultaneously push much higher flows through to discharge at what may be a high backpressure from the downstream piping while still accelerating to operating speed.

--Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[LittleInch]

There is a difference between inrush start current for a motor which assuming Direct On line (DOL) is often 6-8 times FLA for about 3-5 seconds. The motor starters and other electrical equipment knows this and doesn't trip for a set period of time (approx 5 seconds). However in this case, if the motor is undersized, then it can't provide the required torque and power to get to full speed because it is trying to operate at the end of the curve and after the motor start protection drops out, the motor can then trip on excess amps.

So in summary, what that section is trying to say is that you need to have a big enough motor to allow for this start condition and not just the normal running power.

In general it is not good practice to do this (start this way) because running at the end of or beyond the end of the curve is not good for the pump, its seals and bearings or the motor. However they clearly believe that this is how to operate this particular bit of kit and especially if it isn't that big, may be balanced against the cost of doing it another way.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Ahmed Khan]

Thanks a lot all for your inputs

 

[georgeverghese]

See also GPSA for typical centrifugal pump performance curve which shows power demand increase as pump runs out to end of curve if discharge side backpressure is minimal. Valid for fixed speed drives.
This larger power demand on auto start can be avoided if you have a VSD so you can then start at a lower speed and wait till discharge pressure builds back up to normal range. It may be necessary then to check for sufficient level ullage in the upstream source vessel to accommodate the transient level rise while the auto start pump is running at a lower speed for a short time.