The researcher conducted a hotel hot water system project, which provides full-time supply with steam (0.3Mpa) as heat source, water supply temperature of 60-55 degrees, using floating coil heat exchanger , high zone THF1600-LA -Q-5.0, one in use and the other as a backup, low zone THF1000-LA-Q-1.5, one in use and the other as a backup, with the system equipped with safety valves.
When the equipment was installed and tested, the following situation occurred: after the domestic water supply in the low zone was given to the floating coil in the low zone, the safety valve tripped, and the same situation occurred half a minute after the high water supply. Water from the zone was supplied to the floating coil in the upper zone.
The safety valve settings are as follows: Low zone: working pressure at 0.5Mpa, setting pressure at 0.6MPa, readjustment pressure at 0.55Mpa; High zone: working pressure at 0.7Mpa, setting pressure at 0.8MPa, readjustment pressure at 0.75Mpa.
For this problem, I looked for some information about safety valves. A safety valve is an important protection valve widely used in various pressure vessels and piping systems. When the pressure in the pressurized system exceeds the specified value, it can automatically open to discharge excess medium to the atmosphere, ensuring the safe operation of the pressure vessel and piping system and preventing accidents. When the pressure in the system returns to or slightly below working pressure, it can close automatically.
There are several reference values for safety valves: set pressure, reset pressure and discharge pressure.
- The set pressure is the artificially adjusted actuation pressure of the safety valve.
- The reset pressure is the pressure at which the safety valve resets after tripping.
- Discharge pressure refers to the set pressure plus overpressure.
For example: A certain pressure vessel normally operates at 1.0 MPa, the safety valve setting pressure is 1.04 MPa and the readjustment pressure is 0.98 MPa. (The safety valve setting pressure is lower than the design pressure but greater than the maximum system pressure by 1.05 to 1.1 times, and the readjustment pressure should be greater than 80% of the setting pressure . The set pressure cannot exceed the design pressure of the vessel or system).
Therefore, in this project: when the internal pressure of this pressure vessel (taking the low zone as an example) is ≤0.6MPa, the safety valve does not operate; when the pressure is >0.6 MPa, the safety valve trips and the pressure drops. However, when the pressure drops to the normal pressure of 0.5 MPa, the safety valve may not reset and continue to discharge until the pressure drops further to 0.48 MPa (80% of the set pressure), at which point the safety valve will reset and stop discharging steam.
This time the safety valve prematurely tripped, analyzing the following reasons:
1. The mechanical characteristics of the safety valve require that the safety valve reaches the specified opening height during the entire action process, and frequent jamming, shaking and triggering phenomena are not allowed to occur. The occurrence of premature triggering is extremely harmful to the sealing of the safety valve and can easily cause the sealing surface to leak.
The analysis of the reasons is mainly related to the high counter-seat pressure of the safety valve. When the backseat pressure is high, the excessive amount of medium discharge into the container is less and the safety valve has already been replaced. When the operator makes an inadequate adjustment, the pressure inside the container rises quickly again, causing the safety valve to act. In such cases, it can be eliminated by increasing the opening of the butterfly valve.
After opening the butterfly valve further, the source of steam leading to the piston chamber of the main safety valve is reduced, the force pushing the piston down is less, and the safety valve is less likely to actuate, thus preventing continuous starting of the safety valve.
2. According to consultations from construction and installation personnel on site, the working pressure of the safety valve is between 0.3 MPa-0.7 MPa, but the set pressure of the safety valve can only be set to 0. 33 MPa, which obviously does not meet the requirements of setting pressure and counter-closing pressure, therefore it is considered that there is a problem with the quality of the safety valve itself.
3. The jitter phenomenon that occurs during the safety valve discharge process is called safety valve vibration. It is very likely that the occurrence of vibration will cause metal fatigue, reduce the mechanical performance of the safety valve, and cause serious hidden dangers to the equipment. The reasons for the occurrence of vibration mainly include the following aspects:
(1) Improper use of the valve, choosing a valve with too large a discharge capacity (relative to the amount of discharge required), the method of elimination is to make the rated discharge capacity of the chosen valve as close as possible to that required for the equipment settlement value.
(2) Due to the diameter of the inlet pipe being too small, smaller than the valve inlet diameter, or the resistance of the inlet pipe being too large, the elimination method is to ensure that the inner diameter of the inlet pipe is not smaller than the valve inlet diameter or to reduce the resistance of the inlet pipe. Excessive resistance in the discharge piping, causing too much back pressure during discharge, is also a factor causing valve vibration, which can be resolved by reducing the resistance of the discharge piping.
