7 replies to this topic

[Andree]

Dear all,

I need to size a valve, which will be between two different sections of plant containing natural gas. The dP across the valve is 30 barg, I got 0,5 hours to equalise the pressures with an acceptable noise level. How do I calculate the size of the valve and the noise level in dB?

Thanks in advance for your answers.

Kind regards,

FrankSchleck

Frank, you did not provide enough date to anwer your question (e.g. volume of each section).
And calculation of noise level...? This is probably not a Chem. Eng. issue... and I doubt if anybody can do such a calc.

[Andree]

To calculate this you need to know both volumes as well as absolute pressures (not a DP only) to estimate amount of natural gas to flow for pressure equalization).
Generally you can use eqn. of state incl. z (compressibility factor) as for natural gas (some literature data can be found) and then:
1) calculate the amount of gas to flow for pressure equlaization
2) for assumed time of equalization (30 min.) calculate the average flowrate (note: to maintain the flowrate on constant level, the valve opening must increase as DP decreases during equalization)
3) when you know the flowrate then you can calculate the pipe diameter to have reasonable velocity (?) and then select reasonable valve, which enables precise flow throttling and adjustment
(to make kind of characteristic "DP vs. valve opening @ constant flow" you can just use simple Bernoulli eqn. including a drag as a function of valve opening)

[Andree]

Hi Andree,

thanks a lot for your help so far. For a volume V1 = 17,3 m³ and V2 = 24,3 m³, and pressures of 60 barg and 30 barg (both sides can be the high pressure side alternately), I come up with a diametre of ca. 15 mm. Seems a little small.

Kind regards,

FrankSchleck.

yep, it is rather too small - it depends what you assumed as velocity in your calcs...
be aware that when you choose larger pipe dia then you can throttle the flow anyway to adjust velocity and time of equalization to your assumed values, but when you use 15mm ID then you will have limited conditions (provided 15mm is calculated correctly)...

could you tell us what velocity is applied in your calculation (probably this refers to what you expect to guarantee quiet system operation, i.e. low dB as stated in your first enquiry)?

[Andree]

I did quick and rough calcs and in fact for 20 m/s obtained 14.1mm of pipe diameter. It looks to be reasonable value of velocity for pressurized gas flow.
I believe that applying larger pipe with appropriate valve will not have much cost implications and will not constrain you operation if faster pressure equalization is required. Therefore, I would recommend at least 1" NB piping (with schedule as for design pressure for higher pressure vessel)