3 replies to this topic

[peppermint]

Hi all

Would like to hear some advice on the following situation

We have a vertical turbine pump fitted with VFD and on the pump discharge line, there is a NRV followed by a manual hand valve. Intention is to slowly increase operating speed during pump startup so to slowly fill up the empty pipeline, with the hand valve completely open.

There are concerns that pump will reach run-out conditions (at the end of pump curve) while filling up the discharge line; causing motor overload & local pitting.

I understand that this is a major concern for starting pump without VFDs, but not sure if it applies to pumps starting at reduced speed. Any idea?

Many thanks!

[pmunishankar]

Dear,

It depends on the pumps Head, Power vs Flow curve.
Filling up the line is same as discharge line rupture... i.e pressure in the line minimum. If the pump flow is increasing when head is decreasing, it means that the load on motor is increasing. That is the reason whenever pipeline ruptures, pump trips on overload. In your case, your intention is to fill up the pipeline, you can do it by partially closing the discharge valve, thus reducing the load on motor.

In general, Its all depends on PUMP CURVE...

Thanks and Regards,
Muni Shankar

[Art Montemayor]

Peppermint:

What Muni Shankar states is precisely what you should adhere to as the basic, necessary guidelines to a successful pipeline fill – regardless of whether you have a direct-driven pump or a Variable Frequency Drive on the pump. The basic information on your installation is controlled by:

The pump’s characteristics. These are expressed in the centrifugal pump’s Performance Curve that clearly shows that as the pump’s head is lowered, the pumps capacity increases. Using common sense, we know that if your pipeline is to be liquid-filled, it must follow that you have to displace all the gas (or air), that is initially inside the pipe, with liquid. That means you must be continuously venting the displaced gas at the extreme end of the pipeline in order to allow the pumped liquid to enter and carry out the displacement. This also means that the initial head in the pipeline is essentially nothing and that this corresponds to the pump working at a point defined on its performance curve at the extreme end of its curve with infinite flow rate. This can’t be allow to happen because the pump can’t be connected to sufficient horsepower to carry out this defined point. And besides, the pump does not really behave as the end of the curve predicts. Only the pump manufacturer know how the pump will behave beyond a certain point to the right of the design point on the curve. Be aware that the pump curve is not accurate in the area to the right of the design point on the curve.

You need to establish a pipeline full of liquid. And as explained above, it is essestial that you displace ALL of the gas inside it with the pumped liquid. This is best done in a slow, controlled manner. You cannot expect to do it by simply starting the pump and pumping all tha the pump can transport into the pipeline at once. You tend to create agitation and gas pockets as well as running out of horsepower and tripping out the pump.

A VFD device is good – but it has a defined and narrow controlled range of speeds. You cannot expect the VFD to cover ALL the speed range you require to fill a pipeline in a certain time. The past-proven method of throttling the discharge of the centrifugal pump while filling up the pipeline works well. It controls the flow rate through the pump – and consequently its power requirement – and also allows you to define the rate at which you will be venting the displaced gas from the pipeline. Many times, the rate of gas evacuation is what sets the time required to fill the pipeline because it is that rate that has to “controlled” (cleaned, treated, scrubbed, etc.) in order to meet environmental requirements.

[peppermint]

thanks for the insights muni shankar and art montemayor.