Double dry gas seals

Abstract

A double dry gas seal is provided with an additional inert gas circuit (1) whereby the flow of inert gas through the seal may be increased.

Description

[0001] Double Dry Gas Seals

[0002] Field of the Invention

[0003] This invention relates to double dry gas seals and in particular to such seals to be used at higher duties so that volatile emissions can be reduced or avoided.

[0004] Background to the Invention

[0005] Dry Gas Seals (DGS) are used on all manner of rotating equipment such as compressors where rotational speeds and pressure tend to be high. There are several different DGS variants available where each has its merits and limitations. The main types of DGS variants are single, double, tandem and tandem with intermediate labyrinth seals where three of these four variants do not prevent process gases from being vented. The only one of the four that prevents harmful emissions is the tandem seal. Since the seal has an inert seal gas supply the only gas exhausting from the seal is the inert seal gas, usually nitrogen, which has no adverse environmental impact. While this is beneficial, the downside is that the inert seal gas contained within the seal gas cavity is relatively small. This combined with the small amount of gas flowing through the seal faces means that the seal gas gets hot due to frictional heat and viscous shear of the seal gas. A consequence of the heat generation is that a double DGS can only be used on a limited number of duties where the speed and pressure are relatively low. This limits a double seal from being used on a great many centrifugal compressor duties which tend to be high speed and pressure. It follows that if emissions are to be avoided on high duty compressors that a double seal capable of operating at higher duties is required.

[0006] A DGS is typically located at a point at which the rotating shaft passes through the stationary pressurised compressor casing such that the process gas can be contained within the confines of the pressurised casing. In situations where the operating conditions are arduous, sealing is often a compromise since the high loads on the seals and the nature of the process dictate that some gas escapes through the seal, in the form of leakage, for the seal to function correctly. Under certain circumstances, where an inert seal gas source with appropriate conditions can be supplied, it is possible to feed this to the seal so that a process gas at a lower pressure can be completely contained within the casing. This is the basic concept used for double seals where the rotating assembly of the DGS is attached to the shaft and a stationary assembly is connected to the compressor casing. Double DGS’s consist of two sets of faces each having one rotary and one stationary face which are lubricated with seal gas where the interface film thickness between the faces is in the order of 2-3pm. The cavity between the two sets of faces contains the inert seal gas which has to be maintained at a higher pressure than the process gas. On the process side of the seal assembly the faces form a partition between the inert seal gas and the gas in the process side seal cavity. Since the inert seal gas is maintained at a slightly higher pressure, there is a slight amount of leakage of inert seal gas into the process side seal cavity. On the atmospheric side of the seal assembly the faces form a partition between the inert seal gas and the vent system which is normally routed to atmosphere. Since the inert seal gas is maintained at a higher pressure there is leakage of inert seal gas into the vent side seal cavity.

[0007] Environmental concerns have led to the introduction of legislation which is directly aimed at the reduction, or complete elimination, of process gas emissions. One method of achieving this is to install double DGS’s which are fed with inert seal gas. Since the inert seal gas is high pressure and the compressor is pressurised at a lower pressure this prevents process gas emission.

[0008] The primary issue with using double seals is that the small amount of gas contained within the inert seal gas cavity heats up due to frictional effects. Given also that there is low flow (leakage) of inert seal gas through the faces it effectively means that the inert seal gas is stagnant, and the seal temperature will rise as a consequence thus causing damage to the seal. One possible solution would be to restrict the double seal to lesser duties where the compressor operates at a lower pressure and / or speed. However, it would be beneficial for double seal technology to be applied to higher duties. Statements of the Invention

[0009] According to the present invention, there is provided a double dry gas seal, wherein the seal is provided with an additional circuit for inert gas to increase the movement of inert gas through the seal.

[0010] In one embodiment of the invention, the additional circuit includes a portion 1 located outside the seal. This portion may form the entirety of the additional circuit apart from where the additional circuit enters and leaves the main body of the seal.

[0011] A gas booster 5 may form part of the additional circuit where it extends outside the seal.

[0012] In certain situations where the design of the compressor sealing cavity is suitable, it is possible to include the circulating device within or substantially within the double DGS. Provided that there is sufficient radial space periodic features, such as flow modifying vanes, can be provided on the rotating assembly such that motion is imparted to the inert seal gas during operation.

[0013] A seal of the invention may further include a heat exchanger 3 for cooling inert gas within said additional circuit.

[0014] The introduction of a means of gas circulation prevent the inert seal gas from becoming stagnant and an additional heat exchanger 3 also reduces the gas temperature. A combination of one, or both, of the circulating and heat exchange devices allows double seals to operate at higher duties.

[0015] Brief Description of the Drawings

[0016] The accompanying drawings are as follows:

[0017] Figures 1 shows a typical known double dry gas seal;

[0018] Figure 2 shows a double dry gas seal operating at moderate thermal conditions;

[0019] Figure 3 shows thermal issues arising when the seal is operating at elevated conditions;

[0020] Figure 4 shows a first embodiment of a double dry gas seal of the invention; and

[0021] Figure 5 shows a second embodiment of a double dry gas seal of the invention.

[0022] Detailed Description of the Invention

[0023] The present invention will now be described, by way of example only, with reference to the accompanying drawings.

[0024] Figures 1 , 2 and 3 show typical known double DGS’s, the seal in figure 2 being shown operating under moderate conditions and that in Figure 3 under elevated operating conditions.

[0025] Figure 4 shows a double DGS having an additional external circuit 1 for the inert seal gas, the external circuit being provided with a heat exchanger 3 and a gas booster 5. Due to the resultant cooling of the inert gas, the seal is able to at higher pressures without overheating taking place at the seal faces.

[0026] Figure 5 shows a double DGS having an additional external circuit located with the seal providing integral inert gas circulation. Periodic features such as vanes 7 are provided on the rotating assembly such that motion is imparted to the inert seal gas during operation.

Claims

CLAIMS1. A double dry gas seal, wherein the seal is provided with an additional circuit for inert gas to increase the movement of inert gas through the seal.

2. A seal according to Claim 1 , wherein the additional circuit includes a portion located outside the seal.

3. A seal according to Claim 2, wherein the additional circuit is provided with a gas booster.

4. A seal according to Claim 1 , wherein the additional circuit is located substantially within the seal.

5. A seal according to Claim 4, wherein periodic features are located on the rotating assembly to impart motion to the inert gas during operation.

6. A seal according to any of the preceding claims, wherein the seal further includes a heat exchanger for cooling inert gas within said additional circuit.

7. A seal according to Claim 6, wherein the heat exchanger is integral with another component of said circuit.

8. A seal according to Claim 7, wherein said component is a gas booster.

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