10 replies to this topic

[rsk]

total pessure drop for flow through a pipe with CV consists of
1. pressure drop through CV (DPv)
2.frictional pressure drop through pipe.(DPf)
At low flows (DPf)<<(DPv) & a good rangebility is obtained giving precise control.
& CV characteristics most approximates to that of inherent characteristics
also it is desirable to have operaitng region linear range so accordingly Cv is selected.
please open up a discusssion for the same.



Thanks to everyboy
rsk

[fallah]

At low flows (DPf)<<(DPv) & a good rangebility is obtained giving precise control.

The usual rule of thumb is that a CV should be designed to use 10-15% of the total line pressure drop or 10 psi, whichever is greater, so how is your point in this regard?

Thanks

[rsk]

liquid is to be transferred form one tank at 1kg/cm2 to another tank at 20kg/cm2 through a pipe & flow control CV. Now whether CV shall be sized for 15% of 19kg/cm2 overall drop DP. this comes to 2.85kg/cm2 & for the maximum flow, line pressure drop is suppose h. then how the difference ie (DP-2.85-h) is accounted for in the system.
please enlighten

[fallah]

liquid is to be transferred form one tank at 1kg/cm2 to another tank at 20kg/cm2 through a pipe & flow control CV. Now whether CV shall be sized for 15% of 19kg/cm2 overall drop DP. this comes to 2.85kg/cm2 & for the maximum flow, line pressure drop is suppose h. then how the difference ie (DP-2.85-h) is accounted for in the system.
please enlighten

Please specify the pressure in pump discharge (you have to have a pump for described situation).

[rsk]

i regret,
The pump discharge pressure is 10kg/cm2.

[fallah]

i regret,
The pump discharge pressure is 10kg/cm2.

We can not transfer the liquid to destination with 20kg/cm2 pressure, by pump with 10 kg/cm2 discharge pressure.

[rsk]

fallah
i am extremely sorry i mistyped ,Rated pressure at pump discharge is 29 kg/cm2. please guide me

[fallah]

fallah
i am extremely sorry i mistyped ,Rated pressure at pump discharge is 29 kg/cm2. please guide me

Going back to previous posts you have to consider max. 1.4 kg/cm2 pressure drop for CV and lower than 7.6 kg/cm2 (as remaining of totally 9 kg/cm2) for pipe,fittings,...pressure drops.

[rsk]

Going back to previous posts you have to consider max. 1.4 kg/cm2 pressure drop for CV and lower than 7.6 kg/cm2 (as remaining of totally 9 kg/cm2) for pipe,fittings,...pressure drops.
[/quote]

As you say the remaining pressure ie 7.6 to be dropped through pipe,fittings,..., but for the maximum flow my max pressure drop through existing pipesize & fittings is only 5kg/cm2 so i will have to install an orifice for additional drop of 7.6-5=2.6 kg/cm2. Instead if i size CV for 1.4+2.6=4kg/cm2 i will have smaller CV. Now at this point i have confusion i have read that if i design CV for high DP, the required pump head increases but here i dont see similar happening as boundary pressures are already fixed. Please help
regards

[fallah]

If you face with existing pump and pipeline, and also flow set point is corresponding point of H=29 Kg/cm2 on Pump performance curve, then you may manually adjust discharge valve opening on flow set point for approaching to mentioned head.If you didn't finalize the pump providing yet, then you can select pump with lower impeller diameter.

Regards

[djack77494]

I sense that you're "getting hung up" in the numbers. An adequate pressure drop for a control valve is one that allows for effective control. I think that the most commonly discussed situation is for a liquid centrifugal pump system. This is easy to visualize and to use to present concepts. You want your control system to (presumably) vary the flow from the pump to achieve the desired value of your control variable. To do so, you want to push your pump's operating point up and down along its pump curve. My "rule of thumb" is that it should take about 50% of the total hydraulic losses excluding the valve, or 33% of the total hydraulic losses to do that. These are GROSS generalities, and if you don't like them, then go look at some pump curves and come up with your own concept of what it takes. It depends on the shape of your specific pump. It depends on the range of flowrates you anticipate, and the stable range of flowrates your pump can handle. It depends on how much of the pump's work is used to overcome static and superimposed pressures versus how much is used to overcome hydraulic losses. I stand by the 50%/33% as being good generalizations to use when you are far from selecting a pump/knowing its characteristics. Revisit your assumptions as new information is obtained.