When 10% of the maximum allowable working pressure (MAWP) is exceeded, the user may open the rupture disc or the pressure relief valve. If the user is running near MAWP, please consider that due to changes in the pump inverter, unstable flow conditions and thermal expansion of the control valve, surge pressure, pump starting pressure, pump control valve closing pressure and pressure fluctuations may occur.
The first step is to identify the peak pressure during the event that reached the MAWP. If the user exceeds the MAWP, monitor the system pressure 200 times per second (many pumps and piping systems monitor once per second). The standard process pressure sensor will not record pressure transients that pass 4,000 feet per second through the piping system.
When monitoring pressure at a rate of 200 times per second to record pressure transients, consider a system that records the running average in a steady state to maintain the manageability of the data file. If the pressure fluctuation is small, the system will record a running average of 10 data points per second.
Where should the pressure be monitored? Start upstream of the pump, upstream and downstream of the check valve, and upstream and downstream of the control valve. Install a pressure monitoring system at a certain point downstream to verify the wave speed and the start of the pressure wave. Figure 1 shows the pump discharge pressure starting surge. The piping system is designed to be 300 pounds (lbs) American National Standards Institute (ANSI), the maximum allowable pressure is 740 pounds per square inch (psi), and the pump start-up surge pressure exceeds 800 psi.
Figure 2 shows the reverse flow through the check valve. The pump operates in a steady state at a pressure of 70 psi. When the pump is powered off, the change in speed will produce a negative wave, which is then reflected back to the positive wave. When the positive wave hits the check valve disc, the check valve is still open, causing the flow to reverse. When the check valve is closed, there is another upstream pressure and then a negative pressure wave. The pressure in the piping system drops to -10 pounds per square inch gauge (psig).
Now that the pressure transients have been recorded, the next step is to model the pumping and piping systems to simulate the speed changes that produce destructive pressures. Surge modeling software allows users to input pump curve, pipe size, elevation, pipe diameter and pipe material.
What other piping components can produce speed changes in the system? Surge modeling software provides a series of valve characteristics that can be simulated. Computer transient modeling software allows users to model single-phase flow.
Consider the possibility of a two-phase flow that can be identified by transient pressure monitoring in the application. Is there cavitation in the pumping and piping system? If yes, is it caused by the pump suction pressure or pump discharge pressure during the pump trip? Valve operation will cause the velocity in the piping system to change.
When operating the valve, the upstream pressure will increase, the downstream pressure will decrease, and in some cases cavitation will occur. A simple solution to pressure fluctuations might be to slow down the operating time when closing the valve.
Is the user trying to maintain a constant flow rate or pressure? The communication time between the driver and the pressure transmitter may cause the system to search. For every action, there will be a reaction, so try to understand pressure transients through wave speed. When the pump accelerates, the pressure will rise, but the high pressure wave will be reflected back as a negative pressure wave. Use high frequency pressure monitoring to adjust motor control drives and control valves.
Figure 3 shows the unstable pressure generated by a variable frequency drive (VFD). The discharge pressure fluctuated between 204 psi and 60 psi, and the s742 pressure fluctuation event occurred within 1 hour and 19 minutes.
Control valve oscillation: The shock pressure wave passes through the control valve before responding to the shock wave. Flow control, back pressure control and pressure reducing valve all have response time. In order to provide and receive energy, pulsation and surge containers are installed to buffer shock waves. When determining the size of the pulsation damper and the surge tank, it is important to understand the steady state and the minimum and maximum pressure waves. The gas charge and gas volume must be sufficient to cope with energy changes.
Gas and liquid level calculations are used to confirm pulsation dampers and buffer vessels with multivariable constants of 1 at steady state and 1.2 during transient pressure events. Active valves (open/close) and check valves (close) are standard changes in the speed that cause the focus. When the pump is powered off, a buffer tank installed downstream of the check valve will provide energy for the inflation speed.
If the pump runs off the curve, back pressure needs to be generated. If the user encounters pressure fluctuations from the back pressure control valve, the system may need to install a pulsation damper upstream. If the valve closes too quickly, make sure that the gas volume of the pressure regulating vessel can accept enough energy.
The size of the check valve should be determined according to the flow rate, pressure and pipe length of the pump to ensure the correct closing time. Several pump units have check valves that are oversized, partially open and oscillate in the flow stream, which can cause excessive vibration.
Deciphering overpressure events in large process pipeline networks requires multiple monitoring points. This will help determine the source of the pressure wave. The negative pressure wave generated below the vapor pressure can be challenging. The two-phase flow of gas pressure acceleration and collapse can be recorded through transient pressure monitoring. The use of forensic engineering to discover the root cause of pressure fluctuations begins with transient pressure monitoring.
Post time: Nov-15-2021
