Air Inlet Penetration of Two-way Check Valves

During the operation of CRH3 and CRH5 platform EMUs, several times, when the initial lift bow sends power, the braking of individual vehicles fails to relieve the fault, and it is found that the indirect braking pressure is overcharged due to the two-way check valve. In this paper, through theoretical calculation, whole machine and single machine testing and disassembly analysis, four optimization directions are put forward for this valve, and optimization measures are put forward to reduce the risk of two-way penetration of this valve.
 
Principle analysis
 
Formation principle of indirect braking pre-control pressure
Under normal working conditions, when the emergency brake is released, the train pipe is filled with air to 6 bar, and the air storage cylinder B50 is filled with air to 6 bar. At the same time, the indirect brake pre-control pressure on the A3 side of the two-way check valve is connected to the atmosphere through the distribution valve. When the emergency brake is triggered, the train pipe exhausts to 0, and the distribution valve acts to make the air pressure of 6 bar in the air reservoir B50 reach the side of the air reservoir B51 and the bidirectional check valve A3, forming the indirect brake pre-control pressure.
 
According to the ideal gas state equation (1):
P×V=n×r×T （1）
Where: P is the gas pressure; V is the gas volume; N is the number of gaseous substances; R is a constant [r = 8. 314J/(MOLK)]; T is the kelvin temperature. Before and after the emergency brake is applied, the "n×r×T" of the compressed air in the air storage cylinders B50 and B51 and the pipeline do not change, and the volume of the air cylinders and the pipeline do not change. Based on the principle that the product of pressure and volume does not change, it is the formula (2):
P× v1 = 1× v1 after p+2×V2 (2 after p (2)
Where: before P is the gas pressure in B50 before indirect braking is applied (train pipe pressure is 6 bar); Is the volume of v1b50, distribution valve cavity and pipeline (7 l+1.6 l); P 1 is the gas pressure in B50 after indirect braking is applied; V2 is the volume of the pilot pressure pipeline for B51 and indirect braking (1.3L+0.8L); P 2 is the pressure in B51 and the pipeline after the indirect brake is applied, that is, the indirect brake pre-control pressure (the pressure on the A3 side of the two-way check valve). When the emergency brake is applied, the air reservoir B50 and the air reservoir B51 are communicated through the distribution valve, and the indirect brake pre-control pressure is the highest, with 1 after P = 2 after P, which is the formula (3): 6× (7+1.6) = 2×(7+1.6+1.3+0.8) after P, and 2 after P = 4.82 (bar).
According to theoretical calculation and actual monitoring, under normal circumstances, the maximum indirect braking pre-control pressure should be 4. 8 0.1 (bar).
 
Causes of overcharge of indirect brake pre-control pressure
 
When the CRH3 and CRH5 EMUs are powered off, they will trigger the emergency braking of the train pipe exhaust, and the indirect braking pre-control pressure formed on the A3 side of the two-way check valve reaches 4. 8 bar. When the EMU stops for a long time without power, the total wind pressure will inevitably leak, while the volume of indirect braking pre-control pressure is small, and the wind pressure leakage is extremely slow or basically no leakage. At this time, the bow is powered up. When the emergency braking is not relieved, the emergency braking solenoid valve makes the A1 side of the two-way check valve communicate with the total air pressure, which rises slowly with the blowing of the air compressor. When the pressures on both sides of the two-way check valve A1 and A3 are close, the internal movements of individual two-way check valve spools are insensitive, causing the spools to stop at the middle position, and the direct braking pre-control pressure communicates with the indirect braking pre-control pressure, as shown in Figure 2. The pressure on the A3 side of the two-way check valve (indirect braking pre-control pressure) rises to more than 6 bar with the pressure on the A1 side (total air), and then the emergency braking is relieved by operation. The valve core of the distribution valve cannot be moved to the relief position through the air filling of the train pipe (the maximum pressure is 6 bar), and the pressure on the A3 side can never be released, and the relay valve continuously outputs the braking pressure. The fault shows that the braking cannot be relieved.
 
Testing and research
 
Test of the whole machine-test of the faulty two-way check valve on the train set CRH3C and CRH380B(L) platform EMUs are in the emergency system.

In the dynamic state, the indirect braking pre-control pressure formed by train pipe exhaust is 4. 8 bar (on the side of two-way check valve A3), and the total wind pressure outputs the direct braking pre-control pressure (on the side of two-way check valve A1) through the emergency brake solenoid valve. When the total air pressure starts to rise below 4. 8 bar, if the rising speed is slow, the pressure difference on both sides of the two-way check valve cannot be established instantly (the technical standard of this valve requires that the pressure difference on both sides should be at least 0. 25 bar), and at this time, if the valve core is insensitive and does not move in place, the air inlets on both sides will be connected, and the indirect braking pressure will rise with the total air pressure.
 
Select a train of CRH380B EMUs with an overcharge fault of indirect braking pre-control pressure, and monitor the total wind pressure (R), direct braking pre-control pressure (Cv) and indirect braking pre-control pressure (Stv) as follows:
 
When all the air compressors (2 sets) in the train can be started normally, the total air pressure rising rate is about 1 bar/min, and the indirect braking pre-control pressure (Stv) keeps constant at 4. 8 bar, which does not follow the total air pressure (R) and the direct braking pre-control pressure (Cv) rising, that is, the air inlets on both sides of the two-way check valve are not connected, as shown in Figure 3(a); When only one air compressor is started in the EMU, the total air pressure rises slowly, about 0. 6bar/min, and the indirect braking pre-control pressure (Stv) rises from 4. 8 bar to 5. 92 bar following the total air pressure (R) and the direct braking pre-control pressure (Cv), that is, the air inlets on both sides of the two-way check valve are connected.
 
Screening of the whole machine-general survey of sensitivity of two-way check valves in the maintenance warehouse
 
Make a general survey plan of the two-way check valve. The principle of the general survey plan is to reduce the total air pressure to below 4. 8 bar so that the train sets can apply emergency braking so that the A1 side of the two-way check valve communicates with the total air, and the A3 side is the indirect braking pre-control pressure of 4. 8 bar. Only one main air compressor is started in the whole train, so that the total air pressure on the A1 side rises slowly to more than 4. 8 bar, and check whether the indirect brake pressure on the A3 side is overcharged. The specific operation is as follows:
 
(1) After the simulation is stored for a long time, the total air pressure of the train set drops by more than 6. 5 bar initially. Disconnect the main circuit to prevent the air compressor from automatically starting to blow the wind, leaving only one air compressor for standby (other air compressors are cut off). The total wind pressure is reduced to 6. 5 bar by emergency braking, the train pipes are filled with the wind (6 bar), and all air braking of the train set is relieved.
 
(2) Reduce the total wind pressure again through the loop fault isolating switch, and turn the "emergency brake valve" fault switches of all the first cars (car 1, car 00) to the off position (horizontal position), so as to prevent the emergency brake of the whole train from being released due to the insufficient total wind pressure after the train pipe is exhausted, and even make it impossible to operate to reduce the wind pressure. Operate the isolating switch of the "emergency brake circuit" at the non-occupied end for many times, so that the emergency brake circuit is disconnected and then closed, and the brake application is relieved through the circuit, and the total wind pressure is reduced below 4 bar.
 
(3) Check the sensitivity of the two-way check valve.
Restore the "emergency brake circuit" fault switch, restore the emergency brake valves of No.1 and No.8 cars, and apply the emergency brake through the emergency mushroom head. After the train pipe exhausts, the air pressure at the A3 port of the two-way check valve is 4. 8 bar. Lift the bow to close the main circuit, make the air compressor blow up to the total air pressure of 7 bar, and operate the train pipe to charge air to relieve all brakes. If some two-way check valves are insensitive, the indirect brake pre-control pressure Stv will overcharge, which will not relieve the brake. Through screening, it was found that about 1 in every 12 trains (8 short trains, each of which is equipped with a two-way check valve) failed the sensitivity test. By May 2020, 7 two-way check valves had been found and replaced.
 
Single-machine test-bench test
 
To study the conduction of two-way check valves, a sensitivity test bench was set up, and 12 samples of two-way check valves (4 failed valves used in the early stage, 7 faulty valves screened in the whole machine, and 1 new valve) were tested for their functions and the simulation test of train operation conditions.
 
Function: Under the condition of a two-way pressure difference of 0. 25 bar, all tests are passed, and the two-way check valve functions very well. Simulation test of the train set a working condition:
 
(1) Open the shutoff valves H1, H3, HP0, HP1 and HB, adjust the pressure of A1 and A3 ports of the two-way check valve DRV to 4. 8 bar through the pressure regulating valve P, close the HP1 cock, cut off the supply at both sides of the two-way check valve, and keep the pressure at both sides unchanged.
 
(2) Open the H4 cock, adjust the flow rate to make the A1 pressure rise slowly and observe whether the A3 side pressure rises with the A1 side pressure. The test principle is shown in Figure 4.
 
(3) Place the two-way check valve vertically and horizontally, as shown in Figure 5, and check whether A1 and A3 are connected.
 
The test results are as follows: the dimension standard of the two-way check valve is: the outer diameter of the spool is 30c8 (29. 857 ~ 29.89), the inner diameter of the valve cavity is 30h8 (30 ~ 30.033), and the fitting tolerance is 0. 11 ~ 0.176mm. After inspection and measurement, the spool, valve cavity and fitting size of these 12 samples all meet the standard requirements, and there is no abnormality in the mechanical fitting.
 
(2) During the paperback test of 12 sample valves, 11 samples were penetrated on both sides (including the new valve), and only one sample did not appear penetrated; During the vertical test, only the No.10 sample has a penetration.
 
(3) No.10 valve sample has the worst sensitivity. After cleaning and lubricating the No.10 valve, there is still a breakthrough. Although the assembly tolerance is within the normal range, there is slight resistance when the valve core is manually extracted, and no abnormality is found during disassembly, which is determined to be a case problem in the manufacturing process.
 
(4) The test bench can simulate static pressure. For example, the A3 port of the two-way check valve is equipped with a 10 L air cylinder and set at 4. 8 bar, which is basically consistent with the working conditions of the train set; The length of the total air duct of the EMU ranges from tens of meters to hundreds of meters, and the total volume of the air cylinder is 2 700 ~ 3 200 L. Therefore, the pressure rise of the A1 port of the two-way check valve on the test bed is not completely consistent with the flow rate and impact when the total air pressure of the EMU rises, that is, it is difficult to simulate the dynamic pressure situation of the EMU, and the failure of penetration in the vertical installation condition cannot be repeated, but the pressure penetration in the plain installation condition can be repeated, and the same is true of the new valve. From the current test results, the vertical installation method is obviously superior to the paperback installation method.
 
Optimization research direction
 
Optimization of initial power-on operation of EMU
The test is carried out on the EMU with the overcharge of indirect braking pressure. When the total air pressure of the train set is low, emergency braking is first applied, and then emergency braking is applied when the total air pressure rises to 7. 1 bar. At this time, the direct braking pre-control pressure Cv= total air pressure =7. 1 bar, and the instantaneous pressure difference is 2. 3 bar compared with the indirect braking pre-control pressure Stv (double check valve A3 port) of 4. 8 bar.
 
To sum up, after the first pantograph lifting and power transmission of the EMU on the same day, due to the long parking time, the total wind leakage is low. At this time, the emergency braking should be relieved first, and then the pantograph lifting and wind blowing should be carried out so that when the total wind pressure rises to more than 5. 5 bar (the indirect braking pre-control pressure will not be higher than 5 bar), emergency braking and other operations can be carried out, which can avoid the problem of pressure breakthrough on both sides of the two-way check valve. Matters needing attention in the safe operation of EMU's power-on operation optimization scheme;
 
(1) Before relieving the emergency brake, the parking brake should be applied. The parking brake is controlled by the spring energy storage, and there is no wind pressure requirement. It can be safely parked on the ramp no more than 20‰
 
(2) In case of parking on a ramp greater than 20‰ or parking brake failure of individual cars, a spare brake can be applied before relieving the emergency brake to prevent the car from slipping, and the maximum spare braking force should not be less than the emergency braking force.
 
Optimization of installation mode

The failure rate of pressure penetration on both sides of the two-way check valve of the CRH3 platform EMU is much lower than that of the CRH5 platform EMU. The fundamental reason is that its installation direction is vertical, while the two-way check valve of the CRH5 platform EMU is flat. Schematic diagram of two-way check valve vertical installation and flat installation
 
When the pressure of port A3 of the two-way check valve is 4. 8 bar, the pressure of port A1 will increase slowly from the lower pressure until the moment when the valve core acts. The pressure balance of these two installation methods is calculated as Formula (4) and Formula (5):
 
Vertical mounting mode:
PA1×3.14×（b1/2）2+Gb=PA3×3.14×（b2/2）2（4）
 
Paperback mode:
PA1×3.14×（b1/2）2=PA3×3.14×（b2/2）2+Fb（5）
Where: PA1 is A1 port pressure; PA3 is A3 port pressure of 4. 8bar; B1 is 27mm in diameter of A1 pressure acting surface; B2 is the diameter of A3 pressure acting surface 27mm (since PA3 can reach the outer ring of A1 pressure acting surface rubber ring through the valve core side, and the forces of the outer rings on both sides cancel each other out, the actual acting surface diameter of PA3 is 27mm); Gb is the gravity of the valve core of 0.057× 9.8n; Fb is the friction force between the valve core and the valve cavity when it is installed flat (it is determined by the roughness of the oil film and the contact surface, so no quantitative study is made in this paper).
 
Substitute formula (4) and formula (5) to calculate formula (6) and formula (7): vertical installation mode:
PA1=［4.8×105×3.14×（0.027/2）2-0.057×9.8］/
［3.14×（0.027/2）2］
=4.79（bar） （6）
Paperback mode:
PA1=［4.8×105×3.14×（0.027/2）2+Fb］/［3.14×
（0.027/2）2］
=（4.8+0.017Fb）（bar） （7）
 
When the two-way check valve is installed vertically, the valve core can be opened when the pressure at port A1 is 4. 79 bar. For example, the pressure at port A1 rises at a very slow rate, and the pressure at port A3 of 4. 8 bar can pass through the valve core through its very small side (tolerance of 0. 15 mm). The pressure at port A3 drops again, and the pressure difference between port A1 and port A3 increases, which can make the valve core move downward. If the pressure of port A1 rises slightly faster, the pressure difference can be directly formed to make the spool move downward. Therefore, under normal circumstances, the valve core will basically not be overcharged at port A3, which has been verified by the single-machine test in this paper. When the two-way check valve is installed flat, the valve core can be opened only when the pressure at port A1 [(4. 8+0. 017 FB) bar] is greater than the pressure at port A3 (4. 8 bar). For example, the pressure at port A1 rises slowly with port A3 during the movement of the valve core, which eventually leads to the overcharge of port A3. In this paper, all the two-way check valves are verified by a single-machine test.
 
To sum up, this kind of two-way check valve should be installed vertically when it is designed and used.
 
Automatic fault diagnosis optimization
 
When the indirect brake pre-control pressure Stv is too high, the existing vehicle shows that the brake can't be released, and there is no diagnostic code, which is inconvenient for emergency treatment and maintenance in the warehouse. In the new version of brake software, it is planned to add the code "indirect brake pre-control pressure Stv is too high". When the Stv pressure is higher than 5. 6 bar and delayed for 10 s, the brake control system will report a diagnostic fault code to prompt the crew.
 
Routine test optimization
 
Optimize the test bench and test program of two-way check valves, and carry out a sensitivity test on each ex-factory two-way check valve. The newly added test requirements are as follows:
 
The pressure of the A3 port of the two-way check valve (volume 10 L) is kept at 4. 8 bar, and the pressure of the A1 port is increased from 4. 5 bar to 5. 5 bar at the rate of 0. 45 bar/min (refer to the air supply rate of single air compressors of CRH3 and CRH5), and the pressure of A3 port should be kept at 4. 8 bar.
 
Cost analysis and conclusion
 
At present, the two-way check valves of CRH3 and CRH5 EMUs have two-way conduction. The basic reason is that when the main air compressor is started to boost the total air pressure, the two-way check valves have the possibility of two-way conduction, and the slower the total air pressure rises, the greater the possibility of two-way conduction, especially on the two-way check valves with individual differences. This is not the fault of the valve itself or the design problem, but the "special working condition" of the EMU satisfies the condition of two-way conduction of the valve. Therefore, reasonably avoiding this "special working condition" of EMU can completely solve this problem.
 
Through the above discussion, combined with the revision of EMU operation rules, the operation process of EMU initial power-on is optimized, that is, the main air compressor is blown up under the condition of emergency braking relief so that the problem of bidirectional conduction is fundamentally avoided and the cost of modification and renovation is saved; At the same time, the optimization of the installation method should be considered in the initial design of the EMU. The existing EMU is no longer suggested to be changed due to the cost, but it provides a reference for the installation method of the two-way check valve of the subsequent new EMU. Moreover, the optimization of the operation procedures can completely avoid the problem of two-way conduction of the two-way check valve in the paperback mode. Through automatic fault diagnosis and factory routine test optimization, the directivity of the fault is clarified and the screening of individual problems is strengthened.