Do we need the min. flow re-circulation line..?

[nesamani1983]

We have the VSD driven LPG pumps(270 m3/hr & I/L pr=20 O/L pr=70 Kg/cm2 ).

As per the operation seq, disch. valve will start open after 50 secs & speed set has reached. For ex. if 2500 rpm, is set then disch. valve will start open after the pump reaches the 2500 rpm.
Valve opening time is 35 secs.

We feel that running the pump for this 1 min, with disch. valve closed, can create cavitation.so we can install one min.flow recirculation line.
Are we right...? do we need min. flow re-circulation line..?

 

[BigInch]

How are you making cavitation when you have no flow. You are recirculating product inside the pump, not so dangerous if operating time is limited to 1 minute.

I doubt you need a recirculation valve.

It seems like you are doing many things wrong, many things without knowing why and wanting to do even more things, still without knowing why, dreaming up reasons partly based in some things you have heard, but don't understand even well enough to know what causes them, never mind how to prevent them. Isn't it time you found someone that knows what's going on around there?

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky

 

[nesamani1983]

Thank you BigInch.

 

[Pumpsonly]

Since you are running pumps in parallel and with VSD, you should be more concerne about what feed back signal to the VFD. Are you using to control the VSD. Is it flow rate of the main discharge line or individual pump or the pressure? Setting the same RPM for all the pumps may not necessarily give you equal flow for all the pumps even all the pumps are of identical specs and from same make due to construction tolerances.

Are you using 1 common VSD for all the pumps or individually controlled? And why do you need to use VSD?

 

[BigInch]

Givwn that it is taking 50 seconds to reach speed, I'd guess the VSD is "diesel", but how does the driver control affect that problem anyway. In any case, such long times to speed-up are often set as a ramping rate, irregardless of current pressure, or flow rate, to avoid excessive suction pressure reduction, or high discharge transient pressures, should the downstream pipeline pack too quickly. Even a diesel could run up faster than that, so I suspect there is a reason for such a slow ramp.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky

 

[nesamani1983]

Hi Pumpsonly,

We give the input speed in PLC, to get the required flow.
3 seperate VSD for each pump.

 

[CaracasEC]

For a minimum flow recirculation line, check your pump data sheet/performance curve. Is this a parallel operation? And if speed is controlled individually, you're in a big trouble.

 

[BigInch]

Why is it that you're waiting so long to open the valve? Is the suction pressure drawing down to much, or is there very high discharge pressure required even if the pipeline is at minimum flowrate.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky

 

[nesamani05]

Hi BigInch,

No suction pr drwing much.

This is the operation seq set already..But i dont know y so...?

We just give the start command, after setting the speed.
1)Aux LOP start
2) after 30 secs pump speed start to increase & cooler fan starts.
3)after 20 secs MOV start opening.
so total 50 secs.
I dont know y we start the pump in disch. valve closing position.

 

[BigInch]

If you have a high downstream pressure you might have to start closed, or backflow from discharge through the pump might not allow a low torque driver to build speed directly against a high head. That is probably not the case if you have an electric motor, but I would also think you would be very well able to start much much faster. Alternatively, with a large electric motor and low pressure, you may start too fast and kick up some high transient pressures. I could guess either way knowing as little about your system as I do so far. If you want to furnish more details, the guesses might get better. Downstream pressure before start? Downstream pressure after hitting 2500 rpm? Design pressure of pipeline? Pump motor power rating? etc. Maximum pump temperature reached during that 50s startup?

In any case, I wouldn't think that you'd normally get any real problems from that, if you can manage to hold the total time to under 1, or even 2 minutes.

We are more connected to everyone in the world than we've ever been before, except to the person sitting next to us. Lisa Gansky

 

[Pumpsonly]

The operation sequence described in you OP and that of 5 Sept are not quite the same. In your OP you mentioned the valve starts to open only after the pump speed has reached 2500RPM and not time elapsed control.

In you later response it seems it is by time setting.

Actually the MOV start to open 20sec after the the main pump started to turn. The first 30 sec only the aux. LOP is running to pre-lube the pump / motor bearing.
Are you saying that you have to set the speed and hence the flow rate every time before you start the pump operation?

There is a NRV in each of the pump discharge, so starting with opened discharge isolation valve is not an issue even with back pressure.

How often do you have to start/ stop the pumps?

 

[Ork1]

Nesamani,
Just try working with your permissives in your PLC. You can change start-up sequences and or shave off some time at each step. take notes as to pressures and flows etc..