I need to rewrite your question so it makes sense to me. You are asking how the turbine (not compressor) meter can measure CNG flows and pressures by just measuring its upstream static pressure? That’s not exactly what happens. To measure flows using a gas turbine meter, the gas spins the turbine wheel like a windmill which is calibrated to determine the gas’ velocity. The spinning wheel simply reacts to the gas flow and does not compress anything! Now to calculate the flow rate, the only thing missing is the local gas ‘static’ pressure. This is measured upstream because less turbulence exists there from the windmill blades. Using this static pressure, we calculate the gas density, assuming an ideal gas, and then find its mass flow which from the flow continuity equation is defined as: Density x Velocity x Local Flow Area at Pressure Measurement.

The upstream pressure gauge or electronic sensor is usually damped (using some porous material that causes a dynamic but not steady-state pressure drop) so as not to respond to any fluctuations from the turbine blades. This damping eliminates all pressure fluctuations, keeping the pressure rock steady to enable the gauge to perfectly read the steady-state upstream static pressure of the turbine.

Thanks Mr.Vogel

I have to describe my problem as you know the reciprocating compressor at cng stations make pressure pulsation in suction and discharge chamber. I know that turbine meter calculate the upstream flow rate but because of compressor location in downstream of turbine meter we have pulsation that effect the its accuracy. my question how its happen?

Mostafa,

Your english as a second language is good and I am also trying to understand you to help you.

And so, don't feel badly, but I must restate your question so as to accurately respond. You claim that your upstream ‘turbine’ meter, for measuring CNG flow rate, exhibits fluctuating readings that causes inaccuracies caused from your downstream reciprocating compressor pump which is used to create the flow of your CNG?

To correct this you can do one of two things:

1) You must dampen the upstream flow meter’s tiny pressure sensor tube so that the downstream reciprocating compressor pumping actions are not felt. This means you must insert some porous material (like cotton) into the tiny connector tube which transports the CNG gas pressure to the sensor which then reads it. The amount of porous material you stuff in this tiny pipe is trail and error until you stop sensing the fluctuations. Make sure your damping material doesn’t get pushed into your sensor, to plug it up, so have a physical stop barrier.

2) Dampen the main CNG flow duct. Much more effort, not easily done.

A quick way however is to increase the ducting distance between the pump and the flow meter reader. Longer distances will naturally dampen pressure pulses from moving and being felt upstream. However, the longer the ductwork, the more burdened the pump gets to move the required CNG flows. You must find the correct duct length balance that stops pressure pulses yet still lets your pump flow enough CNG.

The hard way is to introduce a Muffler between the meter and pump. The Muffler would be very similar to a car’s muffler which both deadens noise and pressure pulses and provides a consistent back-pressure to keep the engine running smoothly. In your case the Muffler is located upstream of the pump. Remember that your reciprocating pump acts like a car’s engine in creating pressure pulses that needs muffling.