Bud (the senior instrument engineer) and Bob (the new instrument engineer) were on the trip to automate the start-up of the new water injection pump. They caught people's attention because they were wearing coverall clothes and carrying their hardhats on their way from the hotel to the plant.
Bob: Wow, Bud, I wonder what would happen on this start-up. I think it would be great since the simulations you did showed the stuff would work just fine.
Bud: Bob, anything can go wrong in the start-up. There is plenty that can go wrong with this system. The new control system must control the new water injection pump because the existing 2x100% water injection pumps only have design flow rate if 30,000 BWPD. but the actual produced water flows in excess of the 60,000 BWPD. So we need a third one and the pumps will be operated 3x50%. There is a level controller of the of the produced water vessel to control a downstream control valve . Valve travel would have effect on the flowrate, This could result erratic control with poor stability and accuracy.
Bob: I know that the control valve can affect on the pump performances because of the changes in the flow rate. But could you explain it more detail to me Bud?
Bud: Ok, Bob. I will explain it to you a little bit. A fall in the flowrate not only increase the pump pressure but also increase the power consumed by the pump. The changes in the pump pressure may be used as a signal to operate two or more pumps of varying duty, or as a signal to variable speed pump drives.
Effect of valve on pump head and pressure
Bud: There are two consideration during the valve selection for this application
1. If the valve is undersized and installed in the system, the pump would use large amount of energy simply to pass sufficient flow through the valve.But control would be accurate because even small increment of valve movement would changes in flowrate.
2. If the valve is oversized, the energy required for the pump would be reduced with a little pressure drop across the valve in the fully open position.But the large valve orifice would mean that very small increment on valve travel would have a large effect on flowrate. this condition will give poor stability and accuracy.
A compromise is required between the good control achieved againts the reduced energy lost.
We can calculate the valve authority as one of the parameters.It can be determined using equation.
Where:
N = valve authority
ΔP1 = Pressure drop across a fully open control valve
ΔP2 = Pressure drop across the remainder of the circuit
ΔP1 + ΔP2 = Pressure drop across the whole circuit
The value should not be near 0.5 and lower than 0.2
Bob: Ok, Bud now I understand. I hope there won't be any problems that can messed up our start-up.
After a long day. The start-up is fast and smooth. they went back to the hotel
A compromise is required between the good control achieved againts the reduced energy lost.
We can calculate the valve authority as one of the parameters.It can be determined using equation.
N = valve authority
ΔP1 = Pressure drop across a fully open control valve
ΔP2 = Pressure drop across the remainder of the circuit
ΔP1 + ΔP2 = Pressure drop across the whole circuit
The value should not be near 0.5 and lower than 0.2
Bob: Ok, Bud now I understand. I hope there won't be any problems that can messed up our start-up.
After a long day. The start-up is fast and smooth. they went back to the hotel
Excellent information provide for us.....................thanks for sharing
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