Shut Off Head in Pumps

http://cr4.globalspec.com/thread/79019/shutt-off-head-in-pumps

Shutt off head or "dead head" are when the discharge is blocked off or greatly restricted.

I am assuming you know the difference between the two pump types.

Now that shutt off head has been defined what is your intuitive guess on the effect? It is different for both.

Dear Mr. Shrikant45,

Shut-Off Head is the pressure developed by a centrifugal pump, when the delivery valve remains fully closed. In practise we should not close the delivery valve fully and a minimum flow should be maintained which may be in the range of 2.5% to 5% of the capacity of the pump, depending upon few parameters, such as liquid to be handled, temp.of the liquid, viscosity of the fluid, specific heat of the fluid etc.

If delivery valve remains fully closed, CHURNING will take place, inside the pump, and the pump body will get heated as the mechanical energy from the impeller is converted to heat energy.

Positive Displacement Pump delivery valve SHOULD NEVER BE CLOSED, other wise excess pressure developed inside the body will damage the pump.

DHAYANANDHAN.S

PD pumps should always be installed with a Pressure Relief valve or a safety valve in the discharge line PRIOR to the Shutoff or discharge valve. It should be set at a pressure below the max allowable safe operating design pressure of the pump or whatever the lowest safe pressure is for the system it's pumping into. It's often located on the discharge valve manifold to prevent accidents. Therefore your maximum pressure will be whatever the valve limits you to or the driver HP/KW limits you to.

Shut off head of a centrifugal pump is the pressure produced by that pump when the discharge valve is completly closed. The impeller will be spinning in the encased fluid and the motor will be drawing minimum amperage at this point. This is how a centrifugal pump should be started when first put on line provided it's not a multistage, very high pressure application. In a pump such as that tremendous heat will be generated to a dangefous level. Even in a moderate pressure, single stage application the discharge valve should be immediately and slowly opened until the desired volume of flow is acheived and always before 90% of motor amperage is reached. The inlet (suction side) valve, of course, should always be 100% open when the pump is in operation. Throttle the flow only with the discharge valve, never with the suction shutoff valve otherwise there can be major operating problems.

On the other hand closing the discharge valve on a positivwe displacement pump will cause damage to the discharge side piping and likely to the pump also unless there is a pressure relief valve on the discharge side or the pipe bursts relieving the pressure.

Speak with a trusted pump supplier and ask for drawings of both style pumps to give you an understanding of the fundamental differences between the two styles of pumps and their operating characteristics. As an alternative your public libraries will have a Mark's Handbook for Mechanical Engineers in their reference section. Either source will assist you in understanding their operating chacteristics and their differences from one another.

Good luck and check back if you still have any questions.

Lou Bindner

GA,

Another approach is install a tripple duty / check valve, PD transducer and VFD. Do away with the valves all together other than for isolation purposes only.

Thanks Lou.

Well in centrifugal pumps if the discharge valve remains closed and the pumps is still operating it leads to churning of fluid inside the casing and impeller.

Well what are the effects of it on the impeller and casing.

Does it lead to cavitation/vapour formation or any damage to the impeller blades.or what?

It gets hot and what happens then depends on what the casing and impeller are made from. Vapour for sure when it starts boiling.