In this video, we show you how to size a pressure relief valve used on the gas application when the
back pressure is constant.
Let's have a closer look.
A natural gas pipe is protected by a safety valve.
The valve in this application should be able to relieve 5000 and £900 per hour when it opens.
The set pressure is 210 BSI g, the allowable overpressure is 10%.
The back pressure in this configuration is constant and equal to atmospheric pressure at sea level.
The gas is flowing at 120 degrees Fahrenheit with the following properties.
Let's determine the minimum required orifice area for this valve in order to meet these requirements.
The following formula will be used as the fluid is the gas and the flow is expressed as a mass flow
rate in bounce per hour.
Let's determine each parameter.
We begin with this c coefficient.
When the ratio of specific heats is known, the C coefficient can be calculated using the following
formula.
We plug in our variable for a result of 344.
Next.
The relieving pressure is the seat pressure, plus the overpressure plus the atmospheric pressure.
So 210 BSI G plus 10% times 210 plus 14.7 bsi a.
This is equal to 245.7 BPCI a.
Now to determine the capacity correction factor.
Due to back pressure, we need to refer to the following table.
Record.
This table provides standard values of CCB as a function of the ratio of back pressure to relieving
pressure when this ratio exceeds the critical pressure.
Critical pressure here is taken as 55% of accumulated LED pressure expressed in absolute terms.
In our example, the ratio of back pressure to relieving pressure is as follows.
The back pressure is atmospheric, so 14.7 BSI A and the relieving pressure equals 245.7 BSI A as calculated
in a previous step, the ratio is 6%, so way smaller than critical value of 55%.
As a result, Gabe can be taken as one.
Okay.
Next, the relieving temperature expressed in degrees Rankine is as follows.
Lastly, the compressed ability factor of natural gas can be taken as one.
Now we plug in our variables for a result of 0.396 square inches.
Referring back to the API table of standardized dimensions.
We can see here that a g orifice valve with an effective area of 0.503 square inches is the smallest
standard size valve that will flow the required relieving capacity.
Also, it is worth mentioning here that standard materials of construction are satisfactory for this
application, since the flowing fluid is natural gas.
Natural gas is a non corrosive, non-toxic fluid.