By David R. Thornton
This article concentrates on transient vibration caused by rapid closure of a valve in a loading system for a tanker or tank truck. Figure 1 shows a simplified typical flow arrangement of such a loading system for a marine terminal operation. The operation consists of onshore product storage tanks connected to a berthed tanker ship via a pipeline that contains pumping, metering, and flow control facilities. The operational sequence normally consists of establishing flow to the tanker, pumping a measured amount of product to the ship, then reducing flow before closing the valve that connects to the tanker's onboard manifold.
Most jurisdictions have regulations that require that a rapidly closing, emergency isolation valve exist in the piping system to:
Typically, the emergency isolation valve will be a quarter-turn ball valve. If not properly considered, the rapidly closing valve could transform the kinetic energy of the flowing product into the potential energy of a pressure wave that propagates from the valve towards the onshore storage facilities. This pressure wave could result in excess pressure, possibly leading to flange leakage, damage to pressure sensitive equipment, and/or damage to pipe supports and restraints. The pressure wave could also result in significant forces and displacements, particularly if the pipeline contains one or more directional changes. Typically such offsite piping is not provided with sufficient restraint to absorb the possibly large forces. The potential magnitude of the pressure wave when the valve closes can be estimated from the equation:
The following table shows the typical range for the sonic speed in the fluid / pipe combination, the flow velocity, and the fluid density and the corresponding potential pressure increase.
Since the flow in the entire pipeline must be stopped due to the valve closure, the pressure increase propagates in a wavelike manner from the closed valve towards the tankage pumping facilities. As the pressure wave propagates through each segment of piping where a change in direction exists, the pressure wave causes a transient unbalanced force in the piping segment as illustrated in Figure 2.
The magnitude of the force will depend on the amount of pressure rise and the internal area of the pipe. As an example, suppose we have an NPS 10, Schedule Std pipeline transferring gasoline to a ship at a flow velocity of 3 meters per second (9.84 feet per second). The density is about 700 kg/m3 (43.7 lb/ft3) and the wave speed is 1175 meters per second (3855 feet per second) . This gives us a potential pressure rise of 2.4675 MPa (360 psi). The pipe has an internal diameter of about 255 mm (10.04 inches) and an area of 51,070 mm2 (79.16 in2). Thus the potential pressure rise could produce a force along the axis of the pipe of 126,000 N (28,330 lb). Unless the pipe and its restraint system have been designed for this force, it could cause damage similar to that shown in Figures 3 and 4.
However, by altering (i.e., reducing) the closure time of the isolation valve, particularly in the last 15 to 20 percent of its total movement, it may be possible to considerably reduce the magnitude of the pressure rise and the accompanying forces. Alternatively, devices may be installed in the piping system near the closing valve to remove some of the liquid from the pipeline and thus mitigate the effects of the rapid valve closure.
The suitability of a device or action to mitigate the transient pressure and forces cannot be determined by simple equations. Usually, a computer-based study of the piping system must be conducted to determine the complicated time and spatial variation of the pressure, flow, and forces in the system. Such an analysis considers the characteristics of the pipeline components, including the isolation valve and controls or devices used to mitigate the potential for transient vibration. In addition to valve closure, other transient events such as pump start-up or power failure to a pump can also be studied.
Conducting a transient pressure study of a piping system to determine the effects of valve closure requires obtaining information regarding the:
Subsequent to the transient pressure study, sometimes it is also necessary to perform a structural analysis of the piping system for the transient forces generated. This can usually be done using a commercially available piping analysis program such as Caesar II or CaePipe.