Centrifugal Pump Start

I would add the word gradually to point 7.

The pressure in the system depends on the static and friction. Therefore when the second pump is started the pressure will be lower than the combined head and each pump will delver more water than designed for and consume more power. The excessive flow will cause a rapid rise in pressure as the surge progress through the piping and even water hammer.

Note that 2 pumps deliver at a higher pressure but each pump delivers less than for a single pump at the lower pressure.

OK, thank you for reply. One more detail, after stand by pump starting the main will be stopped. The process requires same constant flow. So, could be starded the stand by pump with fully open discharge valve? What is the impact of pump curve against system curve, also the electrical motor load is affected or not?

Keeping the flow constant would require a constant pressure in the discharge system.

I would attempt to open and close simultaneously. (Automating this should be a nice project for a student using a micro controller, steppers and pressure sensors)

Power requirements:

Starting the second pump (closed valve) will increase total requirements.

As the load is switched both pumps will work at a lower efficiency and the total requirements will stay higher until the first motor is stopped.

If controlled the load on each motor wont exceed the running load. (starting current excluded)

This will have to be checked against the characteristics of the units.

To get back to your original question. Maybe CR4 should speculate on the question of switching on the second pump with an open valve and switching off the first pump t seconds later. Fine timing but why not?

Are your pumps of the same size? There are members saying mixing cannot be done but it has been done. After all the pump only see pressure in front of it and not the source of that pressure.

Of course, we are talking about centrifugal pumps from a refinery, same size for main and stand by pump.

More it was a general question of centrifugal pumps start. And what generated my question?

During the last turnaround of my refinery some main position of pumps have been replaced with new ones. Every vendor sent us an operation manual of each pump. As we said before the most recomendations are in same presentation like above.

And for one (for the reason you know we do not discuss about the vendor name ) what was found in manual?

Situation 1: start up with non-pressurized system

- close shut-of valve in the discharge pipe ( minimum flow must be guaranteed );

- start driver and bring the pump rotor immediately to operating speed;

- open discharge valve slowly, until the differntial pressure drops to the value given on the data sheet.

Situation 2: start-up with pressurised system ( precondition is non-return valve in discharge system )

- start driver with open discharge valve and bring the pump rotor immediately to operating speed.

Finish the manual notes.

The pump presented has been started with open discharge valve without to affect anything. And one more thing, on the discharge line it is located a regulator valve to keep constant the flow never mind if are operating main and stand by pump at same time while you change the pumps.

It is possible to apply this rule for all centrifugal pumps or the mentioned vendor selected the power motor in a manner to cover the peak load of starting moment?

I should have asked about the type of pumping and replied thinking of irrigation or bulk water supply. More pumps are started as the flow requirements increase. therefore flow regulators on the discharge side is not used.

I agree that it may work at a refinery.

The non return will operate as a closed valve until pressure is high enough to open the valve.

As soon as the valve opens the pressure will increase because of the flow regulator.

The additional pressure will reduce the flow on the first pump. They will at some stage each deliver 1/2 of the flow at a higher pressure.

Your power requirement for a pumps during switchover should be less than running power.

What do you use for a flow controller?

I would still not do it on irrigation pumps.

Usually in use are pneumatic actuated valve regulators. Normal procedure is to tune the regulator to the desired flow and the regulator will keep the flow value in range limit operating with one pump or both pumps when it is planned any action for stopped pump.

Have you installed a Non Return Valve on the discharge outlet of your pumps?

If not, install one and start pump with discharge valve fully open.

For all pumps are installed check valves on discharge line.

Before any of the mentioned items, suggest you verify the direction of pump rotation, and verify you have liquid to the pump.

I would assume that pump rotation was verified and corrected prior to commisioning the pump (especially in a refinery environment).

Starting a pump with discharge open or closed is a situation where "It depends" is the answer. As Hendrik and Aurel have noted in their discussion, with no back pressure a pump should be starting with the valve cracked open, with sufficient back pressure and checkvalve, it may be started with discharge closed. Very good discussion and comments to outline when and why the discharge conditions should be reviewed to control a pump start requirement.

PLEASE GIVE ME A CLEAR INFORMATION ABOUT THE HIGH CAPACITY CRNTRIFUGAL PUMPS.IS IT ADVAISABLE TO START A HIGH CAPACITY CENTRIFUGAL PUMP BY CLOSING DISCHARGE VALVE,AND WHAR ARE THE PROBLEMS OCCUR.

Capital Means - YOU ARE SHOUTING AT US

TO START A HIGH CAPACITY CENTRIFUGAL PUMP . What Pump? What Capacity? What Head or Pressure?

some of the centrifugal pump not having discharge value only NRV and control value

To answer your question we have to look at the two starting conditions for the pump.

1. Pump running at the EOC

2. Pump start up against the pressurised header (more relavent to the parallel operation & starting of stand by pump on low pressure signal from the discharge header)

Look at the case-1 here typically one can say that when working pump is tripped stand by pump should come online automatically. In such a case pump will run at EOC (not exactly at EOC but you can say close to EOC) momentorily which require enough motor power to take care of the power at EOC.

Now consider the case -2 pumps running in parallel operation. in the start up case when one pump running the discharge header (pressurised header) will be at the rated discharge pressure. the same will act on the second pump & stand by pump (even though NRV is located at the discharge of the pump; from process safety point of view NRV is not considered as safe isolation medium).

Now when we try to start second pump it will have to overcome the back pressure & then it wil take to it's rated speed. to overcome the back pressure motor has to exert higher torque at the start up. that is why we always ask for the superimposed TS curve for pump & motor. You can say the condition as starting against the load.

The same condition exist in normal running condition that when the discharge header pressure is low the stand by pump will come on line which shall have higher stating torque.

So EOC is going with the power & open disharge valve condition goes with the higher starting torque of the motor (refer attached curves which clearly specifies higher starting torque requirement for open discharge valve condition).

In all in the design stage itself designer has to speify whether the pump shall run at EOC or start against the open discharge valve or shall be suitable for both.

Dear Mr. Aurel mares,

Pt. No.5 in your posting,- I do not suggest to crack open the discharge valve, before starting, since DISCHARGE VALVE SHOULD BE KEPT CLOSE at the time of starting of the pump.

After pump reaches rated speed/ changing to Delta Connection THEN ONLY discharge VALVE TO BE OPENED.

Otherwise, crack opening of discharge valve will allow small discharge and it will add additional load to the motor and the starting current will be more than normal, thermal relay or over load relay may trip etc.,etc.

DHAYANANDHAN.S