A reliable surge-protection system must be in place before the occurrence of uncontrolled emergency conditions (e.g., power failure or load rejection in a pump or turbine). The most common tactics to control water hammer can be grouped into three categories, as shown in the following table.
Table 14-6: Comparison of Surge-Protection Approaches
|
Approach
|
System-Improvement Approach
|
Flow-Supplement Approach
|
Surge-Relief Approach
|
| Surge Control Measures/Impacts |
- Realign pipeline route
- Recut or imrpove profile
- Enlarge pipe size
- Reduce flow
|
- Surge tank
- Air chamber
- Increase pump inertia
|
- Various surge-control valves including SRV, CAV, and SAV
- Rupture disk
|
| Reliability |
+ + + + + |
+ + + |
+ |
| Cost |
- - - |
- |
+ + + |
| Operation and Maintenance |
+ + + + + |
+ + + |
+ |
| Complexity |
+ + + |
+ + |
+ |
| Flexibility |
- - - |
+ |
+ + + |
ยท Legend: + Positive effect, - Negative effect
Note: Careful operational procedures and maintenance programs are very important to protect the water system from water hammer damage due to equipment malfunction.
These three approaches differ significantly in terms of the required civil and piping works, physical appearance, hydraulic characteristics, long-term reliability, operational complexity and flexibility, and cost of construction, operation, and maintenance.
However, these measures have a common basis-all three attempt to protect the system from water hammer by reducing the rate of change of flow to minimize the effects of transients. Each approach modifies a different governing parameter, as described in the following sections.
Table 14-7: Governing Parameters for Hydraulic Transients
| A) Piping system characteristics |
(i) Static variables
- Pipe length (L)
- Pipe size (D)
- Pipe profile
- Static lift (H0)
- Pipeline surface roughness (C or f)
- Pressure wave speed (a)
- Pipe flow (Q) or velocity (V)
- Node pressure (P) or head (H)
- Network connectivity (looping, branching, dead ends)
|
| B) Pump-motor characteristics (turbine characteristics are similar) |
- Power (Pw)
- Rotating speed (N) or torque (M)
- Pump total dynamic head (TDH0)
- Pumping capacity (Q0)
- Moment of inertia (WR2)
- Net positive suction head required (NPSHr)
|
| C) Valve characteristics |
- Types (check valve, surge anticipator, vacuum breaker, air release ....)
- Closure characteristics (butterfly, needle ...)
- Operation procedures (time to open, close, operating curve ....)
|
| D) Surge tank characteristics |
- Diameter (DS) or surface area (As)
- Geometry and variation
- Top (spilling) and bottom (dewatering) elevation
- Orifice size and differential ratio
|
| E) Air Chamber characteristics |
- Diameter (Da) and length (La)
- Orifice size and differential ratio
- Orientation (vertical or horizontal)
|
| F) Transient characteristics |
- Upsurge head (Hup)
- Downsurge head ( Hdown)
- Flow (Q) and direction
- Vapor or air volume in line
- Time for maximum transient to occur
- Dampening rate
|
