Pump Startup Sequence

[JimCasey]

My question is about the sequence used in starting a pump.
My customer is using a vertical-shaft pump with a 6" outlet. 800HP motor The outlet piping goes through a check valve and an automated ball valve.Then the line rises 1100 feet.

When they start the pump, the ball valve is closed. They then open the ball valve slowly. It takes over a minute for the valve to reach full open.

The valve is showing signs of cavitation, and the seats are being eroded. I keep wondering why they would start the pump deadheaded, as that MUST cavitate the impeller, too.

Should they not open the ball valve first, then start the pump, allowing the pump to come up to speed initially without load and protected from reverse flow by the check valve? Note again that it is a vertical shaft pump and there is no possibility that it could lose prime.

 

[djack77494]

By starting the pump with the discharge valve closed, the motor's starting current is minimized. You draw the amximum current when starting a motor - often many times more than the "steady state" load. You close the discharge valve so you don't add the additional pumping load to the motor's work of accelerating the at rest rotor.

The pump will be spinning with no flow passing through it for a very short time period using this technique. I've never known this to be a problem. The time period is short - very short if you consider that the discharge valve will start to open very soon after the motor starts to spin. Also, the motor speed is initially low as it must accelerate up to operating speed.
Doug

 

[electricpete]

Starting with discharge valve closed reduces length of starting current draw on some but not all pumps. It will reduce length of current draw on a radial flow pump. It will increase length of current draw on an axial flow pump. Mixed flow somewhere in between but generally tend to act more like radial flow.

There can also be negative water hammer consequences of starting pump with valve wide open.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[JJPellin]

I have one further point. On vertical pumps (specifically, vertical turbine pumps) upthrust can result in shaft buckling if the pump is started up with an open valve and no back pressure. In some of these pumps they have a large number of stages. In order to avoid excessive downward thrust at normal flow rates, they put balance holes through the impellers and wear rings on the back side. At high flow rate, with very little differential pressure, the thrust reverses and the shaft can buckle, knocking out all of the bushings and the mechanical seal.