minimum pressure loss through a control valve 1

[farahnaz77]

How much pressure loss should be take into account for a control valve in piping design step. I know that it depends on control valve type, flow rate,... but maybe there would be a minimum pressure loss in primative piping design where we calculate pressure loss of line. Is there any standard which determine this minimum?
thanks

 

[25362]

One can express the friction loss as the equivalent length of a straight pipe L_e, or by a factor "n" (number of velocity heads, Newtons) by which [ρ]V²/2 has to be multiplied to give the same pressure drop as the particular fitting (valve) in question.

Published data for globe valves:

Opening L_e/D n

Full 330 6
Half 470 8.5

For regular gate valves:

Full 7 0.13
3/4 40 0.8
1/2 200 3.8
1/4 800 15

For a preliminary design basis one generally assumes the valve is one size smaller than the pipe. Factors for both reducers should then be added to those of the valves.

An example taken from the literature:

Line size: 8 inches. A butterfly valve is contemplated with K=3. The reducing fittings account for K=0.29. Thus the total would be K=3.29.
To put the resistance coefficient in terms of the main pipe size, 3.29 is multiplied by (D/d)⁴. In this example:

3.29(7.981/6.065)⁴= 9.87​

K is the resistance factor when the velocity head is expressed in ft, as in u²/2g.

For more information I suggest reading the pertinent chapters on Control-Valve Selection and Sizing by Les Driskell published by the ISA.

 

[jsummerfield]

The application affects the amount of drop allowed for the control valve. Typically when selecting a control valve allow 25% of the system drop. This should include changes in elevation, etc.

The 25% number is for controlability. Steam letdown valves or such applications would take much more pressure drop.

There is an ISA standard (perhaps in the S75 series) that helps to apply the control valve and piping system.

John

 

[friartuck]

In the UK we have something called 'valve authority'.

This is the pressure loss of the valve /pressure loss of the controlled system

The controlled system includes the valve pressure drop

so if you are controlling a heating coil, the authority is the valve drop/(the valve drop + the coil and pipework drop)

The desired authority is reckoned to be good at about 0.3 and really good at 0.5

so if you have a coil at 20kPA loss, if you have a valve at 15kPA and pipework associated with it at 5kPA the authority will be

20/(20+15+5)=0.5

Friar Tuck of Sherwood

 

[itdepends]

I disagree with jsummerfield RE including the elevation change for the control valve pressure drop. By the time you get to sizing the control valve the suction and discharge elevations (and pressures) should be very well defined. The control valve is designed to vary the flow by increasing/decreasing the frictional losses in the pipeline.

The losses due to elevation are fixed (excepting changes in pump suction tank level) and will not change. The losses due to friction/pump efficiency etc. can change.

I'm not explaining this very well but look at the following examples.

Say you have two systems. Both have 100kPA frictional losses but one is with a discharge 10m higher than the suction while the other is 30m higher than the suction

Applying 20% of the total losses for a control vavle would give control valve DP's of 40kPa for the first system and 80kPa for the second system

In reality however given pump curves with similar slopes the 40kPa DP allowance would work for both systems. (approx 25-50kPa is typical for control valve losses on water/dilute slurry systems- howeve it is dependent on your system frictional loss- 15-20% is about right).

The DP should be based on the losses at the maximum (design flow).

As an aside- consider (if possible) the use of variable speed drives- particularly on abrasive fluids/slurries due to lower operating/maintenance costs (power + wear)

Cheers.