Orfice Sizing (Not for Flow Measurement) 4

[RJB32482]

I was wondering what are some applications for orfices except for flow calculation. I believe its for restricting flow for one direction in a pipe. How could one size an orfice for an application like this if you don't know the pressure drop through the orfice (if the orfice is not installed yet)? For example, you want to size an orfice in an 2" ID line that will restrict the flow from 20 GPM to 12 GPM of water. How would this calculation work?

Thank You

 

[briand2]

Here is an example of pressure drop calculations for an orifice:

http://www.efunda.com/formulae/smc_fluids/calc_orifice_flowmeter.cfm

In your particular case, you have too many unknowns. You might not "know" the pressure drop across the orifice, but can pick a value to suit your installation, then calculate the orifice size required for your desired pressure drop at your desired flow rate.

 

[Eltron]

Here's another site that I use all of the time:

http://www.okcc.com

They have the liquid perssure drop equation with all of your needed variables layed out for you. The important thing to know, however, is that a lot of orifice information is gained from empirical measurements and extrapolated to different dimensions.

Hope that helps.

 

[katmar]

RJB,

A typical use would be to restrict flow of cooling water to a particular heat exchanger where several exchangers are piped in parallel. If one of the exchangers has a lower pressure drop than the others, then if the operator opens the water valve fully it could draw flow away from the other exchangers.

Installing a restriction orifice, with its pressure drop designed to give the correct water flow with the valve fully open, in the water line will protect the other exchangers.

It is a crude way of doing things, but I have had to do it several times during commissioning when the actual pressure drops through various pipes and equipment turned out a bit different from the predictions.

If you use a restriction orifice, remember that you are working with the permanent pressure drop and not the pressure drop across the tappings. Make sure that you are using the appropriate equations.

katmar

 

[RJB32482]

Thanks for the info. What would be a good rule of thumb for how much pressure drop would be efficient for a system (say for the system I'm looking at the pressure where the orifice might be placed is 70 psig).

Thanks

 

[katmar]

As far as I know there is no rule of thumb for this type of installation. It all depends on what you are trying to achieve.

Using the example you gave, if you know what the pressure drop is across the system is when you get the 20 gpm, then you can work out what that same system will give with a flowrate of 12 gpm. The difference in pressure drop must be taken up by the restriction orifice.

There are some rules about the ratio of the orifice diameter to the pipe ID, but those are really only for metering orifices, and a bit of inaccuracy is usually acceptable in an RO.

 

[RJB32482]

Thanks for all of the replies. So a formula in the Crane manual states:
Q=236*d^2*SQRT(dP/Kp)

So as pressure drop increases, so does flow rate. So wouldn't adding an orfice (adding pressure drop) increase the flow rate? Or when the statement was said to take the difference of pressure drop, would you take [dP at 20gpm]-[dP at 12gpm] and that answer will equal pressure drop taken up by the orifice?

Thanks

 

[israelkk]

No, what it means is that given an orifice diameter the higher the pressure drop on it the larger the flow rate through the orifice and this is only true with liquids.

With gases the matter is much more complicated.

 

[RJB32482]

Thanks, katmar any other comments??

 

[katmar]

Hi RJB,

I think you are mixing up cause and effect. If there is a higher flowrate then it results in a higher pressure drop. While it is true that they increase together, the way you had worded it made it sound almost as though the orifice was working as a pump.

The reason the Crane formula is cast in that format is that in the way an orifice is usually used, we measure the pressure drop and then we want to calculate the flow. I have not checked this formula - make sure it is for permanent pressure drop if you are designing a restriction orifice.

The second way you described the installation is correct. Assume you have a pressure source that is at 50 PSI. Assuming this won't change much, the full 50 PSI has to be dissipated across your item of equipment. But with 50 PSI you get a flow of 20 GPM, which is more than you want. For the correct flow (you want 12 gpm) you would get only X PSI across the item (where X < 50). So you need to calculate the diameter of an orifice that will "consume" (50 - X) PSI when a flow of 12 GPM passes through it. To do this you just recast the Crane formula to read d = fn(dP,Q). Now you have a total system that gives an overall pressure drop of 50 PSI with a flow of 12 GPM.

 

[stanier]

Why dont you consider flow control valves such as Singer, Cla Val. Over time the reduced energy costs will pay for the valves. Also the process control through your heat exchangers will make for a much more stable and efficient production process.

Rule of thumb is that one third of the system loss should be through the control valve.