Conversion of inches of water to MCFD

Hello guys,

I'm have two analog points. One for Fuel Gas Filter Flow Rate(End Device: flow transmitter) and Fuel Gas Differential Pressure(End Device: Differential Pressure Xmtr). I'm capturing these analog values using a BTR instruction from AB Flex AI card into my PLC-5 over DH+. There is no other parameters involved like orifice size, temperature etc.

How will I write the logic to convert inches of water( reading from DP Xmtr) to a MCFD value that the operators will understand. I think the flow transmitter also sends a value in inches of water that has to be converted to a MCFD value (gas flowrate). Any formulas to do this in PLC-5 ?

Any help is appreciated guys.
Thanks.

If I remember right, flow is equal to some constant (k) multiplied by the square root of differential pressure. You will need to fiqure out what k is, this can sometimes be found on the flow meter, or derived from the ranges on the flowmeter i.e. 0-100in. = 0-1000MCFD.
Obviously, if you are using the above, make sure your analog is actually sending d/p, and not flow rate. My gas prices are high enough already!

Hippie is right if the DP transmitter signal is proportional to differential pressure only. You need to get the constant from the orifice or venturi supplier.

However, many DP transmitters will do the hard work for you. You can set them to provide a signal proportional to the square root of the DP, and scale them so that 4 mA = 0 flow and 20 mA = the flow at the max DP range of the transmitter. By knowing that k factor you then know what the max flow at that DP is, and you are in business. Your 4-20 mA signal is now linear and proportional to flow.

I know you say "there are no other parameters involved", but since you are using DP, there has to be a primary flow element and that primary flow element was 'sized' for certain conditions, which determine the flow rate at whatever DP.

The differential pressure is created by a "primary flow element": an orifice plate, a venturi, a pitot tube, a wedge, or any number of other devices.

The manufacturer of the primary flow element supplies a "sizing sheet" with each flow element that describes what the dp is for a given flow rate at certain design conditions. These design condition include working "static" pressure (line pressure) and temperature for gases, because of the compressability of gas.

As others have pointed out, if the transmitter provides a signal that is the square root of the dp, and you know the max flow rate for the dp at the transmitter's 20mA value, then you'll have a linear 0 - flow rate signal over the range of 4-20mA. But that assumes you know what dp the primary flow element was designed to create at a given flow rate.

For orifice plates, the DP drop can be calculated from the bore size of the orifice plate (engraved on the handle), if the line pressure is known and a temperature assumed, for common gases.

Many primary flow elements come tagged with some form of ID that might be used to obtain sizing info.

Dan

Your request for a conversion from inches of water to MCFD is missing a step.

What you are looking for is a total: millions of cubic feet per day.

You need a flow rate to calculate a total.

DP by itself is not a flow rate. DP has to be converted to a flow rate, like dubic feet per minute or cubic feet per hour, before you can total it.

Inches of water represents some flow rate, but that flow rate depends on the sizing of the primary flow element used. You need the sizing data, or you need to back calculate it, for the primary flow element.

Dan