CHAPTER 14 - MEASUREMENTS IN PRESSURE CONDUITS
15. The Pressure-Time Method
The pressure-time data recorded during downstream closure of valves, wicket gates, or other devices can be used to calculate discharge (Gibson, 1923). Gibson developed elaborate equipment and detailed computational procedures for determining discharge. Modern pressure cells, electronic computers, and recording equipment have made the technique easier to accomplish. Codes such as those published by the ASME (1992) have been established for turbines. The pressure conduit should be at least 25 ft long, preferably longer.
Discharge is computed from:
(1) Pressure-time recording taken while the valve or other control device is closing.
(2) The cross-sectional area of the conduit.
(3) The length from control device to the entrance.
This method has recently been applied to shorter, low head turbines (Almquist et al., 1994). The pressure variation is automatically recorded with respect to time on equipment especially devised for this method. Use of the Gibson method requires specially trained personnel. Patents restricting the use of the equipment and method have expired.