Mechanical seal issues with the cryopump in low-temperature methanol washing?
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I would like to ask the technical experts: in the low-temperature methanol washing process, ethylene glycol is often used as the sealing flush fluid for the mechanical seals of many low-temperature pumps, such as methanol circulation pumps. However, the melting point of ethylene glycol is around -19°C, while the temperature of the liquid entering the dimethanol circulation pump is -23°C. Will using ethylene glycol as a sealing fluid cause it to freeze in this case? Additionally, the ethylene glycol used as the flush fluid needs to be cooled by water after passing through the storage tank. I don’t understand why cooling is necessary since the boiling point of ethylene glycol is around 190°C – is it perhaps to prevent it from vaporizing?
The temperature of ethylene glycol is relatively high as it circulates within the seal; the heat comes from the heat dissipation of the mechanical seal, and this is why cooling water is necessary. Ethylene glycol and methanol do not come into contact unless there is a leak in the primary seal.
Reply to 2# lxm082: Hello, the methanol circulation pump is used to handle low-temperature methanol; therefore, the temperature of its mechanical seal should be quite low. If the mechanical seal generates a lot of heat during operation in such low-temperature conditions, why not use water for cooling instead of ethylene glycol? I think ethylene glycol is used because it has good fluidity at low temperatures. This also indicates that the temperature of the mechanical seal in the methanol circulation pump must be very low. I’m not sure if my reasoning is correct
The mechanical seal of the low-temperature methanol washing pump uses an internal balanced series seal design. For sealing flushing, the PLAN11+52 scheme is employed; ethylene glycol is used as the flushing fluid for the second seal. The melting point of ethylene glycol mentioned by the original poster, around -19°C, refers to pure ethylene glycol. Mixtures of ethylene glycol can remain liquid even at around -60°C, so an inlet liquid temperature of -23°C for the dimethanol circulation pump will not cause the ethylene glycol sealing fluid to freeze. During operation of the pump, friction at the mechanical seal generates heat. The glycol used as the flushing fluid is cooled by cooling water through a reservoir, in order to prevent the temperature of the glycol from rising. If cooling water is not used, the temperature of the glycol will increase; although it won’t vaporize, this could still lead to the failure of the mechanical seal (as it exceeds the allowable operating temperature for that seal). But as far as I know, the seal fluid in the mechanical seals of ordinary low-temperature methanol washing pumps does not require cooling with water from a storage tank, because the heat generated by friction in the mechanical seals is not sufficient; moreover, due to heat transfer with the medium being transported, the temperature rise of the seal fluid should remain within the operating range allowed for that mechanical seal Experts with practical experience are also invited to share their opinions on the above points!
Reply to 3# scg311: It is absolutely not permissible to use water to directly flush the mechanical seal; before starting the machine, the mechanical seal must first be connected to a flushing fluid, with the pump chamber filled with the medium to be transported. In other words, there is a process prior to startup, and during this time heat transfer causes the water flush liquid in the seal chamber to freeze rapidly, which is dangerous!
Reply to 5# mjaline: If what you say is true, and the ethylene glycol used as the sealant in the second set of seals isn’t pure ethylene glycol, then what concentration of ethylene glycol is used as the sealant during normal production? Thank you.
Reply to 6# scg311: It shouldn’t be a pure ethylene glycol solution. As for the exact concentration, I lack practical experience and can’t give an accurate answer; it’s recommended to consult the manufacturer of mechanical seals or seek guidance from someone with relevant experience!
Our company’s cryopumps use methanol as the flushing fluid for mechanical seals
Generally, 50% ethylene glycol is mixed with 50% water, resulting in a freezing point of --41°C.
Our company specializes in seals imported from Germany, focusing on seals designed for extreme temperatures: ultra-low temperatures of -270°C (for use with LNG, liquid nitrogen, liquid hydrogen, and liquid helium), as well as ultra-high temperatures of +1100°C. They are also suitable for high pressures up to 680 MPa, and offer excellent corrosion resistance. We provide both dynamic and static seals; the dynamic seals can handle speeds of up to 40 m/s (or even higher). For more information, please contact 18678860293 via VX