How to specified pressure drop for orifice and control valve sizing 4

[devaxrayz]

Hi all,

I am doing sizing of orifice and control valve. One of the data required for the sizing is pressure drop( or inlet and oulet pressure).How do I specified the pressure drop for this sizing?? (The orifice or CV is not there yet so i dont know the inlet and outlet pressure).

Is there any rule of thumb for specify pressure drop across orifice or control valve.

FYI: Fluid of service is water.

 

[katmar]

The orifice should use as little pressure drop as possible, subject to the pressure transmitter being able to measure it accurately. Check with the instrument engineer who will specify the transmitter what pressure drop s/he needs to provide the accuracy you require. A reasonable starting point would be 2 or 3 PSI.

The rough rule of thumb for the CV is that it should use about 30% of the system pressure to give good control. But this varies with the system, particularly the ratio between the static head (which does not vary with flow) and the friction losses (which do vary with flow).

 

[gmorin]

Keep in mind that the pressure drop across a control valve is generally dicated by the upstream and downstream pressures, not vice versa.

Unless your pressure source and user are also being designed as part of your project, the pressure drop across your valve / orifice system will be your pump (or header, reservoir etc.) pressure minus whatever pressure is needed for the service downstream of the valve, minus pipe friction losses, gravity losses, etc.

Figure out what the Cv needs to be under your governing conditions (normal / min / max) and you can size the valve appropriately.

What is the purpose of the orifice? Is it for flow measurement as assumed by katmar, or is it to protect the valve from cavitation???


Greg

 

[jsummerfield]

The fixed range of antique mercury manometers established 100-inch water column as the normal basis for the orifice pressure differential using flange taps. This is a good starting point, even though you can buy transmitters ranged 0-1 inch water column.

John

 

[BSQUAREDBUCKEYE]

Wow how many times have I been asked that question. Control valve people live very simple lives, "What is the P1 value at the inlet of the valve, and what is the P2 value needed at the outlet of the valve?" Now it's up to the Mechanical Engr. sizing the pump and the Piping Designer to figure out the rest.

As you know centrifical pumps have a curve that indicates pressure relative to flow. The lower the flow the longer the fluid can be in the pump impeller to convert the kinetic energy to potential energy in the form of pressure. As flow goes up, discharge pressure decays. The other thing that occurs is that as flow goes up piping friction goes up exponetionally. You need to identify the minimum and maximum flowing conditions and calculate the head loses(2.31 feet = 1 psi)and the frictional loses of the piping(reference books, ie. for a GPM so many psi loss per 100 feet of pipe size)and equipment(heat exchangers(refer to their OEM data sheet etc.)

Those P2 values you find by just the opposite above. Where does the fluid have to go, up a hill? Well you better have enough pressure right after the valve to overcome that head pressure needed, remember 2.31' = 1 psi.. Does the fluid have to be pushed into a tank under pressure at the top of hill? Now you need that P2 value to be greater than the elevation and the tank pressure and the frictional piping loses. OK?

You need to determine two flow conditions; 1)What is the minimum pressure drop we allocate to the control valve at the maximum flow condition? (Min dP @ Max. Q = Max. Cv) 2)What is the maximum pressure drop we allocate to the control valve at the minimum flow condition? (Max. dP @ Min. Q = Min. Cv) The first condition will tell you HOW BIG a valve you need, the second condition will tell you WHAT KIND of valve you need. Remember the pump generates higher pressure at low flow and piping losses are small at low flows so the valve has to dissapate all that energy. This is where "flashing" and "cavitation" occur. These conditions can destroy valves in hours.

Although water was indicated as the media, there was no mention of temperature. Temperature effects the fluids vapor pressure. Closing a valve to increase pressure drop will reduce the flow area which will accelerate the fluid. As kinetic energy increases, potential energy decreases and the fluid's pressure falls below it's vapor pressure boiling the fluid. As the area increases again down stream of the valve and the fluid velocity slows back down it may or may not rise back above it's vapor pressure. If it does it will collapse the boiling bubbles and at one bubble diameter will yeild the adjacent metal ie "Cavitation". If it doesn't then it will just flash but in either case the flow is choked and no increase in dP across the valve will increase flow.

Now if you actually read this far here's the reality. 1)The Inst. Engr. would prefer 30% of the system allocated to the valve. It will have a better "installed characteristic" for them to tune their loop. The Mechanical Engr. buying the pump will say "No way!" "I am not buying a bigger pump and motor, consuming electricity, to dissapate it over a control valve." "Only 10% can be allocated." So 20% allocated to the control is a good rule of thumb. Sometimes the system dynamics can be so complex that it would take weeks to calculate all the conditions. 2)Locate the valve as close to the pump as possible or at the lowest point in the system or the coolest condition in the system. The first two of these are trying to increase the P1 and P2 values as high as possible above the fluid vapor pressure. The third is to try a have the fluid's lowest vapor pressure.

Rearding the orifice plate; if it is for flow measurement, a repretable instrument supplier can provide the bore calculation given the pipe size, schedule, flow, temperature and media based on a 100" H2O span. Also ask for the calibration chart.

If it is to be used with the valve to distribute pressure drop be very careful. An orifice plate will generate the pressure loss in only a very small range of the valve's total range.

Metso Automation, formerly Neles-Jamesbury has a sizing program that will allow you to size valve with restriction plates. It will confirm these findings. You should be able to obtain it for free on their website.