Starting pumps against high head 3

[CarNut37]

Hi - I'm from Aussie so excuse the SI units. We have a pump system design due for construction later this year. Duty is seawater/mud slurry at 20% solids pumped to a dam 8km distant, with 9km piping rising 112m from pump centreline. I was always of the opinion that you started centrifugal pump systems by starting the pumps against a closed isolation valve and opening the valve slowly, hence accelerating the pipe contents slowly until full flow is obtained. Does the same apply for a (full) system like this with 112m of head already "on" the pump (There is a check valve then isolation valve at the beginning of the main line). For the 750m3/hr normal flow, 2 Slurry 10/8 end suction pumps are used in series. The first is fixed speed, the second VSD. Do we start pump 1 then ramp up pump 2, then open the valve, or open it after pump 1 is started. Any tips ?

 

[quark]

From your post it seems 112 meters is the statc discharge head. So you should go with normal pumping practice. If you provide postitive suction of 112 meters then you can start the pump with valve open.

The idea of closed discharge starting is to overcome inertia quickly and control starting torque (so initial current) at the same time. Mind that the pump has to support 112 meters of water column if the discharge valve is opened before starting.

You should not have problems if you start the VFD operated pump even after opening the discharge valve provided you match the ramp up time.

Regards,

 

[d23]

CarNut37:

Ask your manufacture and provide them the best data you can, but we start pumps with both system and downhole check valves all the time. I have pumps with over 5000 feet (1524 meter) delta head starting like this all the time on D-O-L or VSD's. No problems as long as your NPSH(r) is OK.

Watch sea water and oxygen. It's hell on equipment. Sea weed is too.

Good luck!

 

[PUMPDESIGNER]

There are many factors governing use of a control valve.
However I am afraid that most of what I hear and read concerning starting of a pump against a head is dead wrong and based on a misunderstanding of hydraulics.

d23 is correct, no problem starting against head.
Why?
Simple, centrifugal and axial pumps do not care what their internal pressure is (as long as the case can withstand it), they just produce a pressure differential. It does not matter what the internal pressure is, 50 psi or 50,000 psi, the impeller spins just as easily.

As far as blaming pressure surge on pump starting, that of course can happen, but most people do not get that correct either. Pressure from a pump does not create pressure surge, but rather, velocity changes cause pressure surge. Actually, having that head against the pump on startup could be like a closed control valve. Pump starts and builds pressure against a back pressure, water starts to move, good deal.

In fact, depending on exact system and pump parameters, across the line starting without VFD could be great.

PUMPDESIGNER

 

[scalleke]

Can I bring Npsh issues into the discussion. The two pumps are installed in seres, first the straigtht on line and second the VFD driven unit.

If the second pump is started first it will have the first pump in it's suction line as a restriction. Also the second unit is VFD so presumably to make up the missing head that the first unit cannot deliver. Why have the second unit start up and try to deliver a really high head if this will not be the actual job of the unit.

My vote goes for starting up the first unit first DOL. Once discharge pressure is established start up unit 2 and settle VFD so that speed of unit 2 is such that just slightly over the system pressure is delivered then open the discharge valve.

Starting up the VFD unit first will make it hunt its curve too much in search for the right operating point.

Best regards
scalleke