1.0Line sizing for gas piping | pressure drop across line in gas systems

Line sizing for gas piping | pressure drop across line in gas systems– You you can my previous articles on how to line sizing, how to do pressure drop calculation for liquid and vapor line.

This article covers line sizing for gas piping, schedule number calculation for gas piping, pressure drop calculation for gas piping, how to calculate gas density? What is ideal gas law and how it useful in line sizing calculation.This is a help full article for piping design engineer.

How to calculate line sizing for gas?

There are two types of fluid one is compressible fluid and second is non-compressible fluid.

what is compressible fluid?

Compressible fluid are those fluid who get compressed if we apply pressure on it. Compressible fluid are those fluid who’s density is get changes with respective pressure.

Air, Nitrogen and oxygen are compressible fluids, LPG, etc.

what are the non-compressible fluid?

Non compressible fluid density dos nit changes with pressure. Non compressible fluid are water, oil, etc.

Data required to calculate line sizing

To calculate line size for gas or liquid we need some data which is necessary to do calculation.

First required data is flow. First we need to know how much flow we want to transfer. To calculate flow you need knowledge about process, like material balance of process. If you know the flow in terms of mass like kg/hr then we need density to convert the mass flow in to volume flow. We can calculate the density using ideal gas law PV= n RT.

Another required data is allowable or permissible pressure drop. Permissible tf drop is nothing but maximum allowable pressure drop fir process.Another required data is pressure at source.

Source pressure is necessary to decide velocity of gas flow. Depending upon pressure we need to assume velocity. Another thing required is pipe moc. Pipe moc is necessary to check roughness as per pipe moc.We need viscosity of gas in cp.

Appart from process data we need mody chart to calculate friction factor.

Ideal gas law

Ideal gas law state that Pressure is directly proportional to the temperature. It means if pressure increases then temperature also get increase.

P * V= n * R * T,

where,

V is volume

T is temperature

p is pressure

n is number of miles, I. e. Given weight / Mole weight

R is gas constant.

Modifying ideal gas law we can calculate gas density using following formula,

gas density = P * Molecular weight / gas constant * temperature

Note :- gas comes under compressible fluid category. So it’s density changes with as per pressure changes.

To do line sizing for non compressible fluid read our article on it. There is no big difference in line sizing procedure for compressible fluid and non compressible fluids.

Line sizing for gas piping

Now we first take an overview on steps which are necessary to do line sizing or to do piping design.

• Assume velocity as per process pressure in m/sec, in general we take 5 m/ sec per kg pressure. Means 5 m/ sec for 1 bar pressure and 10 m/sec for 2 bar pressure for gas system.
• if flow is not in m3/ sec then first convert the flow in m3/sec. For this purpose you need density of gas. If you don’t know the density then you calculate it from using formula, Density = pressure x molecular weight / R x T.
• Calculate the required diameter of pipe using formula, Q = velocity x cross sectional area.
• calculate Re number using formula, Re = Dvp/u.
• Using moddy diagram calculate the friction factor.
• Calculate pressure drop using formula, Delta P = density * L * Friction factor * velocity^2/ (2*g*diameter)
• check the obtained dela P as per permissible pressure drop.
• if obtaining pressure drop is matches calculate pressure drop then our calculation are ok.
• if calculate pressure drop is high then take low velocity and do all calculation again.
• In next article we will come Line sizing for gas piping | pressure drop across line in gas systems.

Conclusion

Above article explained that how to calculate required line size for compressible fluid. There is little difference in of line sizing for gas and liquid.

Thanks for reading..,……

if you need calculation sheet for kine sizing please comment your email address I will share.

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Compressible and non compressible fluid