Safety valve installation instructions and precautions analysis Safety valve critical pressure ratio study – Lecco valves

Safety valve installation instructions and precautions analysis Safety valve critical pressure ratio study – Lecco valves

Safety valve installation instructions
In petrochemical plant design, as the number of medium and high pressure levels of equipment and pipelines involved in the increase, the use of safety valves has increased accordingly. Therefore, the safety valve correct, reasonable layout is particularly important.
1. The safety valve on the equipment or pipeline should be installed vertically and as close as possible to the protected equipment or pipeline. However, the safety valve of the liquid pipeline, heat exchanger or container, when the valve is closed, the pressure may rise because of thermal expansion, can be installed horizontally.
2, the safety valve should generally be installed in the place where it is easy to repair and adjust, and there should be enough working space around it. Such as: vertical container safety valve, DN80 below, can be installed on the outside of the platform; DN100 is installed outside the platform near the platform, with the help of the platform can be used to repair and overhaul the valve. And should not be installed at the dead end of long horizontal pipes to avoid accumulation of solids or liquids.
3. The safety valve installed on the pipeline should be located in a place where the pressure is relatively stable and there is a certain distance from the fluctuation source.
4, the safety valve to the atmosphere, for the general harmless medium (such as air, etc.) discharge pipe mouth is higher than the discharge port as the center of the 715m radius of the operating platform, equipment or ground 2.5m above. For corrosive, flammable or toxic media, the discharge outlet should be more than 3m higher than the operating platform, equipment or ground within a 15m radius.
5, the safety valve outlet is connected to the pressure relief pipe, to be inserted into the pipe from the upper side down to 45 Angle, so as not to pour the condensate into the branch pipe, and can reduce the back pressure of the safety valve. When the constant pressure of the safety valve is greater than 710MPa, insert 45 must be used.
6. There should be no bag-shaped liquid in the discharge pipe of the wet gas pressure relief system, and the installation height of the safety valve should be higher than that of the pressure relief system. If the outlet of the relief valve is lower than the pressure relief main line or the discharge pipe needs to be raised to access the main line, a liquid storage tank and a level gauge or manual liquid discharge valve should be set at a low and easily accessible place, and be regularly discharged to the closed system to avoid accumulation of liquid in the bag-shaped pipe section. In addition, in cold areas, the bag pipe section needs steam heat to prevent freezing. The steam tracing tube can also vaporize the condensate in the bag tube to avoid accumulation of liquid. But even if the use of heat tracing tube, manual drain valve is still necessary.
7, the safety valve outlet pipe design should consider the back pressure does not exceed a certain value of the constant pressure of the safety valve. For spring type safety valve, the general type of the back pressure should not exceed 10% of the rated pressure of the valve, bellows type (balanced type) the back pressure should not exceed 30% of the pressure of the safety valve, for pilot type safety valve, the back pressure does not exceed 60% of the constant pressure of the safety valve. The specific value should refer to the manufacturer’s sample and be determined by process calculation.
8, because the gas or steam is discharged into the atmosphere by the safety valve outlet, the opposite force is generated on the center line of the outlet pipe, which is called the reaction force of the safety valve. The influence of this force should be considered in the design of the outlet line of the relief valve. Such as: safety valve outlet pipe should be provided with a fixed support; When the inlet pipe section of the relief valve is long, the pressure vessel wall should be strengthened.
Safety valve operation precautions
1. The safety valve using department should clearly put forward the following safety operation requirements for the safety valve in the process and post operation rules:
1. Operation process indicators (including working pressure, working temperature or low working temperature, setting pressure);
2. Safety valve precautions and operation methods (for safety valve with wrench);
3. Items that should be inspected in the operation of safety valve, possible abnormal phenomena and preventive measures, as well as emergency disposal and reporting procedures.
2. Regular inspection should be carried out during the operation of the safety valve. The inspection period is formulated by each user according to the specific situation, and the length should not exceed once a month. The following items should be inspected in particular:
1. Whether the nameplate is complete;
2. Safety valve seal is intact;
3. Whether the cut-off valve used with the safety valve is fully open and the seal is intact;
4. Check whether any exception occurs during operation.
5. Whether it can take off flexibly when the setting pressure is exceeded in operation.
Three, the safety valve in the process of use, when the following problems occur, the operator should report to the relevant departments in time according to the prescribed procedures:
1. Overpressure does not take off;
2. Do not return to the seat after taking off;
3. Leakage occurs;
4. Before the safety valve cut off valve and safety valve seal fall off.
Four, the pressure vessel in the process of operation, the safety valve before the cut off valve should be in a fully open position and seal. It is strictly prohibited to jack the safety valve to death, cancel or close the cut-off valve. Any change in safety valve operation must be approved by the supervisor.
Five, the safety valve with pressure work, is strictly prohibited to carry out any repair and fastening work. Need to carry out repairs and other work, the user unit should formulate effective operation requirements and protective measures, and the technical person in charge of the agreement, in the actual operation of the door must send people to supervise the site.
Six, the operator is prohibited to open and remove the lead seal or adjust the safety valve setting screw.
7. Spare safety valve should be properly kept and maintained.
Study on the critical Pressure Ratio of Safety Valve – Study on the critical Pressure Ratio of safety Valve – Lyco Valve Abstract: A formula for calculating the critical pressure ratio of safety valve is presented.
The TEST RESULTS SHOW THAT THE CRITICAL PRESSURE RATIO OF THE SAFETY VALVE IS MAINLY AFFECTED BY THE CRITICAL PRESSURE ratio OF THE nozzle and the DISC flow resistance coefficient, and because the disc flow resistance coefficient is too large, the safety valve is generally in the SUBcritical flow state.
Gb50-89 “Steel Pressure Vessel”, according to the flow state of the safety valve is different, put forward two kinds of displacement calculation formula, therefore, to judge whether the safety valve is in the critical flow state or subcritical flow state, is the premise of correct selection of displacement calculation formula.
At present, there are two views on the value of the critical pressure ratio of the safety valve: ① it is considered that the critical pressure ratio of the safety valve is the same as the critical pressure ratio of the nozzle in the specifications of various countries, and its value is 0.528 [1,2].
② Many experts and researchers believe that the critical pressure ratio of the safety valve is less than the critical pressure ratio of the nozzle, and its value is about 0.2 ~ 0.3 [3] So far, no rigorous and accurate theoretical calculation method of the critical pressure ratio of the safety valve has been accepted.
Therefore, determining the critical pressure ratio of safety valve and correctly judging the safe flow state is still an urgent problem to be solved in engineering, which has not been reported in the literature so far.
Through theoretical analysis and experimental study, the author discusses the flow state of safety valve and puts forward the theoretical calculation formula of critical pressure ratio of safety valve.
1 Safety valve critical pressure ratio critical pressure ratio RCR refers to the ratio of inlet and outlet pressure when the airflow velocity reaches the local speed of sound at a small flow passage section.
The critical pressure ratio of the nozzle can be calculated by the formula in theory.
When the nozzle inlet pressure ratio is lower than or equal to the critical pressure ratio of the nozzle, the disturbance of the outlet inlet pressure ratio cannot exceed the sonic plane due to the sonic flow on the outlet section, so the disturbance cannot affect the flow in the nozzle.
The airflow pressure on the outlet section remains unchanged at P2 / P1 = Cr, the airflow on the outlet section is still sonic flow, and the relative displacement remains unchanged, namely W/Wmax=1. At this time, the nozzle is in a critical or supercritical flow state [4].
In addition to the nozzle, the critical pressure ratio of other structures often need to be determined by test, and the critical pressure ratio determined by test is called the second critical pressure ratio for distinction.
Due to the complexity of the safety valve structure, it is difficult to determine the flow velocity at the small flow passage cross-sectional area of the safety valve, so it is impossible to determine the critical pressure ratio of the safety valve accurately according to whether the small flow passage closure area reaches the speed of sound.
At present, the method to determine whether the safety valve has reached the critical flow state is to measure the displacement coefficient of the safety valve. It is believed that the safety valve will reach the critical flow state as long as the displacement coefficient does not change with the pressure ratio [3].
The measured results show that the displacement of the safety valve always changes with the change of the pressure ratio, but when the pressure ratio of the safety valve is lower than 0.2 ~ 0.3, the variation of the displacement of the safety valve with the pressure ratio is small, and people think that this small change is caused by the measurement error, so it is judged that the critical pressure ratio of the fully open safety valve is about 0.2 ~ 0.3.
The theoretical basis of this test method for determining the critical pressure ratio of the relief valve is that the pressure ratio disturbance cannot exceed the sonic plane in the critical and supercritical flow state, so that the relative discharge rate of the nozzle remains unchanged
However, in the state of critical or supercritical flow, the flow at the nozzle outlet section is sonic flow, resulting in the relative displacement
As the inlet pressure P1 of the safety valve increases, the disc resistance pressure drop P increases, and the outlet pressure P2 of the nozzle in the valve also increases. As a result, P2 and P1 may increase step by step, resulting in the pressure ratio of the nozzle in the valve r= P2 / P1 gradually to a fixed value.
As can be seen from the calculation formula of nozzle displacement, the nozzle displacement gradually becomes a fixed value, and the displacement of safety valve changes little or unchanged with the pressure ratio.
However, this does not mean that the flow velocity at the small flow passage section of the safety valve reaches the local speed of sound. Obviously, the pressure ratio at this time is not necessarily the critical pressure ratio of the fully open safety valve.
Moreover, when the opening height of the disc is small, the displacement coefficient of the safety valve does not change with the pressure ratio even when the pressure ratio reaches 0.67. Of course, this pressure ratio cannot be considered as the critical pressure ratio of the safety valve, for theoretically speaking, the critical pressure ratio of the safety valve cannot be larger than the critical pressure ratio of the nozzle.
Figure 1 safety valve structure diagram and the theoretical calculation model by figure 1 b shows that the relief valve and its ideal equivalent nozzle is reflected in the difference between a disc resistance pressure drop p because of various specifications of traditional displacement calculation method adopt the ideal equivalent nozzle model calculation, and ignore the effect of disc resistance pressure drop, which will easily confused the relief valve and nozzle, This may lead PEOPLE TO BELIEVE THAT THE CRITICAL PRESSURE RATIO OF THE RELIEF VALVE IS THE SAME AS THAT OF THE NOZZLE, 0.528, WHEN IN FACT THE RELIEF VALVE AND THE nozzle are clearly different.
The main difference between the safety valve and its ideal equivalent nozzle is reflected in the disc resistance pressure drop, while the traditional calculation model does not consider the role of the disc resistance pressure drop P, which is unreasonable.
The theoretical velocity of nozzle expressed by static parameters is [5] : 3) Where, K is the adiabatic index; A1A2 is not the valve nozzle inlet and outlet of the flow channel section; R0 gas constant; T1 is the inlet temperature; R is the pressure ratio at the inlet of the nozzle in the valve, and r=2/ P1. Now divide both sides of Equation (1) by P1 and substitute equations (2) and (3) into the simplified formula, and the relationship between the pressure ratio of the safety valve and the pressure ratio of the nozzle in the valve can be derived as follows: In Formula (4), pressure ratio of safety valve B, RBB /1 Since the critical flow passage section of the fully open safety valve is at the nozzle throat, the critical flow state * of the safety valve can be reached at the nozzle throat.
According to Equation (7), the critical pressure ratio RBCR of the safety valve is mainly affected by the critical pressure ratio RCR of the nozzle and the disc flow resistance coefficient F.
When the DISC flow resistance coefficient F increases, THE critical PRESSURE ratio OF the safety valve will decrease because the critical pressure ratio of the nozzle is constant.
It can be seen that the critical pressure ratio of the safety valve decreases with the increase of disc flow resistance coefficient.
When the flow resistance coefficient increases to a certain critical value, the critical pressure ratio of the safety valve will be reduced to zero.
If THE DISC RESISTANCE COEFFICIENT EXCEEDS THIS CRITICAL VALUE, THE VALVE CANNOT REACH THE CRITICAL FLOW STATE BECAUSE THE DISC FLOW RESISTANCE coefficient is TOO LARGE, and the safety valve is completely in the subcritical flow state.
Therefore, if there is a critical flow state in the safety valve, the critical pressure ratio of the safety valve should not be less than zero, that is, when RBCR ≥0, the disc flow resistance coefficient should meet F ≥2/ K.
For air, k=1.4 and F ≤1.43.
Thus, if the safety valve is in a critical flow state, its disc flow resistance coefficient F cannot exceed 1.43.
In order to determine whether the safety valve is in a critical flow state or a subcritical flow state, the author conducted tests on the disc flow resistance coefficient of two kinds of safety valves, A42Y-1.6CN40 and A42Y-1.6CN50. FIG. 2 shows the test relation curve between disc flow resistance coefficient and the pressure ratio of the safety valve, in which H is the full opening height and Y is the test opening height.
The test results show that the disc flow resistance coefficient of the fully open safety valve is more than 1.43.
Therefore, it can be concluded that even if the inlet pressure of the safety valve is large, the safety valve cannot reach the critical flow state due to the valve disc resistance pressure drop is too large, so the safety valve is generally in the subcritical flow state.
In order to prove the reliability of this inference, the author has tested the pressure ratio of the two safety valves and the pressure ratio of the nozzle in the valve, and the test results of the pressure ratio of the safety valve and the pressure ratio of the nozzle in the valve
The test results show that when the inlet pressure of the relief valve reaches 0.6Pa gauge pressure), the pressure ratio of the nozzle inside the two valves is more than 0.7.
It can be seen that the nozzle in the valve should be in a subcritical flow state.
The critical flow passage section of the fully open safety valve is at the nozzle throat, and the critical flow state of the safety valve * can be reached at the nozzle throat.
Therefore, when the nozzle inside the safety valve reaches the critical flow state, the safety valve is in the critical flow state.