Change Over of Screw Pump

I see two choices: 1) Start the second pump while the first is still running; 2) Stop the first pump and then start the second.
Which is better for your situation; zero flow for a brief moment, or double flow for a brief moment?

Thank you sir,

Is there any chances of increase in pressure w.r.t. flow if the first is still running because it is a sreew pump.

The Screw Pump is a PD Pump and the flow would almost be constant irrespective of pressure unlike the centrifugals.

As the pump is delivering the flow the pressure at outlet (again assuming that it is below the set pressure of different protection devices) would be directly dependent on the flow (the output restriction would be almost the same)

And that in simple term means that the pressure would rise almost to double.

This is the condition where the protection mechanisms should come into picture and the simplest one is the pressure relief valve.

Teh LOS (Lube Oil Systems) that we have designed (and procure from our suppliers) for the equipments we manufacture has two protections

a) The screw pumps are specified with a built in relief valve that actuates at almost double the working pressure.

b) One external relief valve at the common outlet header line that is set at somewhere around 1.5 times the working pressure.

All the sub-components (filters, on line coolers, pipe lines, equipment...) are ensured to be OK at 2.5 times the working pressure.

If you are a user then the last sentence is something you might not be aware of but the external relief valve if available is going to take care of the extreme eventuality.

BTW - ensure that the protection mechanism doesn't make the system trip at over pressure when both the pumps are on.

<...chance...>?

It is a certainty.