Page 217 - Buried Pipe Design

Design of Pressure Pipes 191

Figure 4.4 Valve stem travel versus flow stoppage for a gate valve.

The effective time is the time that should be used in water hammer
calculations. Logan Kerr 14 has published charts that allow calculation
of the percent of maximum surge pressure obtained for various valve
closing characteristics.
There is one basic principle to keep in focus in the design and oper-
ation of pipelines: Surges are related to changes in velocity. The
change in pressure is directly related to the change in velocity.
Avoiding sudden changes in velocity will generally avoid serious
water hammer surges. Taking proper precautions during initial fill-
ing and testing of a pipeline can eliminate a great number of surge
problems. In cases where it is necessary to cause sharp changes in
flow velocity, the most economical solution may be a relief valve. This
valve opens at a certain preset pressure and discharges the fluid to
relieve the surge. Such valves must be carefully designed and con-
trolled to be effective.
Surge tanks can also be designed to effectively control both positive
and negative surges. In general, they act as temporary storage for