Helium mass spectrometer leak detector manufacturer: Calculation of allowable leakage rate
The sealing performance of the equipment or product is relative to the allowable leakage rate. Different devices require different functions and different air tightness requirements, so the leakage rate allowed is also different. This allowable leakage rate is usually designed by personnel through careful consideration and calculation according to the equipment’s requirements for air tightness. The allowable leakage rate is not only the main design index of equipment with air tightness requirements, but also the basis of detecting equipment leakage.
So, how to calculate the allowable leakage rate value? Here are some typical cases.
(1) Allowable leakage rate of dynamic vacuum system
The so-called dynamic vacuum system refers to the system that the pump is still pumping during the working process, such as vacuum melting furnace, vacuum coating machine, particle accelerator, etc. Dynamic vacuum systems typically use pumps that pump faster to capture and maintain vacuum. Therefore, due to the large leakage holes on the system, it can still work, so the requirements for air tightness are correspondingly reduced. However, once the system is separated from the pump, the vacuum will drop dramatically. The allowable leakage rate of the system can be calculated by the gas flow stability relationship. Under no-load conditions, when the system reaches dynamic equilibrium, the amount of gas entering the system space per unit time must be equal to the amount of gas taken out by the pump. The average pressure at this point is the limit pressure p0 that the system can reach. According to the relationship of flow stability, there is a relationship of flow stability Q=Sp0 Q= air flow, including two parts of leakage and venting, that is, Q=QL+QFS= the effective pumping speed of the pump to the system.
When designing a vacuum system, if the effective pumping speed S of the system is determined by the pump, the system is required to reach the limit pressure p0
It is also known that the gas flow [Q] allowed by the system is [Q]≤ Sp0
Generally, 1/10 of [Q] is selected as the allowable leakage rate [QL]. Then, the allowable leakage rate of the dynamic system should be:
[QL ]=1/10[Q ] ≤1/10S.P0
[For example] a coating machine, the effective pumping speed of the evaporation chamber is 100L/s, and the required limit pressure p0 is 1×10-5Pa, then the allowable leakage rate of the coating machine
(QL) = 1/10 (S * p0) = 1/10 * 100 * 1 x 10 x 10-4-5 = 1 (Pa L/S) = 1 x 10-7 (Pa, m3 / S)
(2) Allowable leakage rate of static vacuum system
The so-called static vacuum system, that is, the vacuum system that has been isolated from the pump when working, is generally known as a closed container or closed device, such as a picture tube, electronic tube, etc.
The holding point of the static system is that the volume is small, the required limit pressure is low, and the vacuum degree can still meet the working requirements for a long time after being isolated from the vacuum pump, that is, the life is long. Therefore, the requirements of this system for leakage and venting are very high. Assuming that the pumping effect of the degassing agent added in the closed system is not considered, the pressure of the device during isolation is P0, the Z high pressure required to ensure the normal operation of the device is Pt, the cavity volume of the device is V, and the storage and working time of the device after isolation is t, then the total allowable air load [Q] (gas leakage and venting) of the device should be [Q] ≤V×(P0-PT)/t
Take device allows the leakage rate (QL) to 1/10 of the [Q], the [Q L] [Q] = 1/10 or less (0.1 * V * (P0 – Pt))/t
[For example] For an electron tube, the cavity volume is 0.1L, the pressure p0 when sealed is 1×10-4Pa, the Z high pressure pt when the electron tube is working normally is 1×10-1Pa, and the device storage and working time t is required to be 10000h. Then, the allowable leakage rate of the tube should be
[Q] L acuities were 0.1 * V * (P0 – Pt)/t = (0.1 * 0.1 * (1 x 10 x 10-4-2-1)) / 10000 x 3600 = 2.8 x 10-10. Pa L/S