4 replies to this topic

[go-fish]

Is a control valve always required in pump discharge or is it only required only when there are alternate operating conditions requiring flow control.

I have an offsites pump which is just transfering oily water from one tank to another. There are no alternate conditions. Initially, source tank is full and destination tank is empty. After pumping, its vice versa. My understanding is that even when there is no control valve in discharge, the pump will travel along its performance curve by itself and adjust its head and flow depending upon the level in the destination tank.

[ankur2061]

go-fish,

The basic nature of the centrifugal pump is that for a given pump configuration the flow rate adjusts acording to the resistance it faces. Essentially it means that your operating point for the pump shifts on the Flow versus Head curve depending on the system resistance. Higher the resistance (head), lower the flow & vice versa for the same pump configuration.

Control valves on centrifugal pump discharge are provided for specific applications such as precise forward flow control (FCV) to the destination, level control (LCV) of the source vessel, minimum safe continuous flow control to prevent flow to fall below the specified minimum value, combination of pressure & flow control with pressure control (PCV) in the minimum flow recirculation line and flow control (FCV) in the forward flow line. Other combinations are also possible depending on the application.

On a discontinuous service where you are just pumping out the contents of one tank to another a control valve may not be required. However, I would definitely provide a low pressure switch on the pump suction to trip the pump in order to prevent the pump running dry (cavitate) when the source tank level falls to very low levels.

Hope this helps.

Regards,
Ankur.

[demank]

If the source tank is a storage tank (no maintaining the level at specify condition of source tank), of course this flow control (LCV) is not required.
Otherwise the source is Pressure Vessel such as 2-phase separator which need maintaining the liquid level at specified condition, the flow control need installed.

*agree with ankur, the low level switch need installed on the source tank to prevent pump run dry.

[go-fish]

Thanks for the detailed explanation.

[kkala]

Is a control valve always required in pump discharge or is it only required only when there are alternate operating conditions requiring flow control.

To the previous contributions, following may be worthwhile noting.
1. In refinery offsites there are a lot of pumps without control valve at discharge, as in your application.
2. A centrifugal pump starts with closed discharge valve, progressively (e.g. in several seconds) opened by operator after the start up of the pump.
3. I have seen automatic min flow installed nowadays for all pumps, including offsite pumps, as a general policy.

Edited by kkala, 18 May 2010 - 04:10 PM.