How do I Calculate Flowrate , Q, for a one half inch pipe diameter air line using 120 psi gauge ? 3

[FacilityMan]

I have a tunnel compressed air system that delivers air to to my facility at 120 psig. I have a compressed air work station with 1/2 inch piping and an R12 Norgren pressure regulator that drops the 120 psig air pressure to 30 psig. Cv is 3.6 for the regulator. Downstream of the Norgren Air regulator is a 1/4 inch diameter kunkle model 548 relief valve set at 30 psi with 48 SCFM capacity.
HOW do I calculate the flowrate in the 1/2 inch diameter pipe at 120 psig ? and How do I know if the Relief valve is sized correctly ?


Joseph Muniz
Facility Engineer

 

[bimr]

You can use a flow chart to determine the capacity of the pipe:

http://www.engineeringtoolbox.com/air-flow-compressed-air-pipe-line-d_1280.html

The 1/2" compresses air line will deliver 50-60 SCFM.

 

[LittleInch]

First though you need to know the ID of the pipe, the length, any other restrictions and what the pressure is at the start and end points.

Note that a lot of the charts and simple equations work on the principle that the gas (air) is essentially incompressible. Once your end pressure starts to be less than 90% of the inlet pressure they become more and more unreliable as the expansion of the air takes place generating significant additional velocity.

Flow capacity using CV is easy to calculate - search for gas CV equation.

If max volume in exceeds relief valve flow out then it is undersized.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[FacilityMan]

Thank You Gentlemen ! I showed the Building Engineer the Compressed Air Flow Capacity Curve Chart and he did not think the chart was applicable to our situation. Since he is the "owner's Representative" I accept his decisions. For a 1/2 inch diameter copper pipe at 120 psig, chart shows approximately 90-98 SCFM. The Building Engineer stated several reasons why the chart would not be accurate to the table full of people. So no acceptance on the Compresses Air Pipes Chart.

Joseph Muniz
Facility Engineer

 

[racookpe1978]

Well, if you (if they) reject that chart, then I guess I need that room to provide a different chart and validate it.

Now, what is the source of the pressure into the relief valve? (The "proper" size of the relief valve is one size that can release at least as much, if not more, than the maximum pressure coming into the pressure vessel. Your relief valve needs to be large to continuously release all of the 120 psig air coming through that 1/2 line that is ????????? long?

 

[bimr]

FacilityMan, you are not reading the chart properly. The chart is a log scale.

If you don't like the chart, use the app at the bottom of the page:

"Related Mobile Apps from The Engineering ToolBox

Compressed Air Pipe Lines App"

The app shows 14.8 psi pressure loss over 100 feet when compressed air is flowing at 40 SCFM

Another calculator:

http://us.kaeser.com/Online_Services/Toolbox/Pressure_drop/default.asp

 

[hfgnick]

Without going into detailed calculations, the basic issue here is that the relief valve has to be sized based on the maximum flowrate due to regulator failure in the wide open position. When this happens sonic gas flow will exist in the bore or port of the regulator. This will occur since sonic flow occurs when the ratio of upstream pressure to downstream pressure is less than about 0.5 (See API 520 and 521 for explanation of critical flow pressure) since there is enough differential pressure energy available to get to sonic flow conditions (pressure head is converted to velocity head and enough pressure head is available to achieve sonic flow velocity). In this case the upstream pressure is 134.7 PSIA and downstream is 44.7 PSIA. Using the wide open Cv value and inlet pressure along with the required properties of air there are compressible gas flow valve equations that can calculate the maximum flowrate at sonic (critical) flow conditions. Then you can use API 520/521 relief valve sizing equations for the calculated flowrate.

Also you can estimate the maximum flow by assuming a perfect frictionless orifice (= minimum port size of regulator valve) at critical flow (sonic) conditions and use the ideal gas flow equation PV = mRT to calculate the estimated ideal flowrate (actually mass flowrate to be solved for here). Where P = critical sonic flow pressure PSIA x 144 (approx. 0.5 x upstream absolute pressure for air) V = volumetric flow rate cu. ft./sec. (= Velocity x Area of port = sonic velocity ft/sec x area ft. sq. = <SQUARE ROOT gkRT> times area of port PI(D2)/4 ft. sq., where g=32.2 gravity constant, k=1.4 ratio of specific heats for air, R=1545/29 universal gas constant for air, T sonic flow temperature = T upstream times 2/k+1, degrees Rankine = deg F + 460), m = mass flow rate lbs/sec(unknown to be calculated), R = 1545/29, T = T upstream x 2/k+1 = Sonic flow temperature deg. Rankine. Solve for mass flow rate then use ASME/API relief valve equations with calculated mass flow to size relief valve based on 44.7 psig inlet pressure. Make sure all the units you use in the equations are consistent (ft., Lbs, sec, deg R, ft sq. PSIA x 144 to PSFA, etc.