Pipework flow losses

Gas pipework in a user s premises serves the function of transporting the gas from the meter to the point of use in a safe way and without incurring an avoidable pressure loss. For low-pressure installations, the permitted pressure loss is only 1 mbar from the meter to the plant manual isolating valve at maximum flow rate. The pipework must be sized adequately to allow for this. Boosters are sometimes used to overcome pressure losses, but the use of a booster should never be considered a satisfactory substitute for correct design of pipe sizes. Where gas is available at higher pressures it may be permissible to tolerate pressure losses of more than 1 mbar. 

This example was calculated under the assumption that the friction in the pipe was negligible, and that all the frictional pressure drop occurred across the valve. In the normal plant situation, however, significant friction losses are incurred as the gas flows through the pipework. This complication to the process of finding the flow is dealt with by a variety of methods in Chapter 10. 

Another scheme which makes duty control difficult is that often installed on high viscosity fuel oil systems. To prevent pipework blockages, such fuel needs to be kept above a minimum temperature. Should its flow drop, heat losses from the pipework can result in the temperature falling below this minimum. To ensure a flow is maintained, even if a heater is shutdown, fuel is circulated around the site via a heated storage tank. The pipework passes alongside every heater and each burner on the heater can have its own take-off. Since it is not practical to measure the flow to an individual burner, flow meters are installed on the supply to and return from the heater. Fuel consumption is then determined by the difference between these measurements. However, because consumption is small compared to the circulating flow, the calculation is very prone to measurement error. For example if the supply flow is 100 %, measured to 2 %, and the return flow is 95 %, also measured to 2 %, then the calculated consumption could vary by a factor of nine, i.e. from 1 % to 9 %. Such a measurement cannot be used as a DV. 

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