GB2297589A - Mechanical seals - Google Patents

Abstract

A double mechanical seal 6 and barrier fluid gear pump system includes a gland housing, which receives a rotatable shaft 3, with the seals defining between them a cavity for barrier fluid. An inlet and outlet are provided in the cavity. A barrier fluid circulating gear pump 10 is coupled to the rotatable shaft 3 so as to derive its motive power from the rotatable shaft. The system includes a header vessel 7 for storing barrier fluid, an inlet pipe 8 for directing barrier fluid from the header vessel 7 to the double seal 6 and an outlet pipe 9 for recirculating barrier fluid back to the header vessel 7. The gear pump 10 is attached to the inlet pipe 8 adjacent to the header vessel 7.

Description

MECHANICAL SEALS This invention relates to improvements in mechanical seals.

Double mechanical seals are commonly placed at the interface between a process pump and the rotatable shaft which drives the pump. The double mechanical seal is there to avoid loss of fluid from the pump in the area where the rotatable shaft is inserted into the process pump. The rotatable shaft is usually powered by a motor.

Most double mechanical seals have a cavity defined by the sealing faces, the gland housing and the rotatable shaft through which a barrier fluid is circulated to support the correct running of both sets of faces by cooling the seal.

The barrier fluid is stored in a header tank and circulated to the seal by means of inlet and outlet pipes. At present, there are two main systems for circulating the barrier fluid. The first makes use of a thermosyphon and the second a separate circulating pump.

The thermosyphon system allows for heat to be removed from the seal faces by the circulation of water. As the water is heated, it expands and thus becomes less dense than the incoming, cool water. Placing the water outlet from the seal cavity above the inlet ensures that the heated water is ducted out of the seal cavity and escapes back to the header tank. As a result, cool water is drawn in through the water inlet.

It is sometimes preferable to use oil rather than water as the barrier fluid, for example where the product being sealed is incompatible with water. Because oil does not expand sufficiently to thermosyphon when it is heated, it has to be pumped around the system. Barrier fluid also has to be pumped where large amounts of heat have to be removed from the seal, for example where the equipment is being used with explosive chemicals in which the build-up of heat could be extremely hazardous, or where a pressure differential is required to ensure that the barrier fluid is on seal faces and not the product.

In these circumstances, a second motor has been used to drive the barrier fluid pump. However, the use of a second motor can be problematical in areas where there are explosive chemicals, because the propensity of the motors and their electrical connections to cause electrical sparks can be a fire hazard. Furthermore, the use of additional pressurizing pumps has historically been extremely expensive, because they are used in hazardous chemical environments and are therefore required to seem stringent safety requirements. The header tank itself, used in the pressurized system, must also be manufactured to ASME VIII standard.

It has been proposed to avoid the necessity of using a second pump and motor by incorporating fins onto that part of the seal which is attached to the rotating shaft or otherwise modifying the shape of the seal cavity to as to allow flow to be induced by the rotation of the shaft.

However, it has been found that such designs are rather less effective than might have been hoped and may not perform well enough for critical hazardous chemical systems in that whilst they create a limited flow, they do not generate enough positive pressure to effect a pressure differential across the seal faces.

The present invention avoids the need for a second motor yet provides an arrangement which allows effective circulation of the barrier fluid and the generation of a variable positive pressure differential. The arrangement comprises: a double mechanical seal including a gland housing adapted to receive a rotatable shaft so as to define between them a cavity for barrier fluid; an inlet and outlet for barrier fluid communicating with the cavity; a pump for circulating the barrier fluid; and means for coupling the rotatable shaft to the barrier fluid pump so as to power the pump from the rotatable shaft.

The arrangement may further include: a header vessel for storing barrier fluid; an inlet pipe for directing barrier fluid from the header vessel to the double seal; an outlet pipe for recirculating barrier fluid back to the header vessel; and a pump attached to the inlet pipe adjacent to the header vessel.

Preferably, the pump is a gear pump.

The header vessel is preferably above the double mechanical seal. The pressure of barrier fluid inside the header tank should preferably be lower than that in the inlet pipe.

The inlet and outlet pipes may be finned.

The gear pump may, for example, be driven by a continuous flexible drive element (such as a belt or chain), one end of which passes around a driven wheel (such as a pulley or sprocket) which provides drive to the gear pump and the other end of which passes around a driving wheel (such as a pulley or sprocket) which takes drive from the rotatable shaft.

The driving and driven wheel sizes may be chosen to provide a desired rate of barrier fluid flow and/or pressure.

The arrangement of the present invention will now be described with reference to the accompanying drawings.

Fig. 1 is a side view of the arrangement.

Fig. 2 is a front view.

As can be seen from figs. 1 and 2, the arrangement includes a pump 1, bearing house 5 and motor 2 all mounted on a bed plate 12. The bed plate 12 may be metallic, e.g. steel, or formed from an aggregate such as concrete. Its purpose is to prevent any vibration from one part of the system from being propagated to other regions. The process pump 1 is driven by the motor 2 by means of a rotatable shaft 3 connected to the motor 2 by a drive couple/clutch 4. The rotatable shaft passes through the bearing house 5 after which it connects with the pump 1.

A double mechanical seal 6 surrounds the rotatable shaft 3 where it enters the pump 1. The double seal 6 is typical in that it includes inboard and outboard seal faces and a gland housing which, together with the shaft, define an internal cavity through which barrier fluid is allowed to circulate. The barrier fluid is stored in a header vessel 7 and is directed to the double seal 6 by means of a downward inlet pipe 8. The inlet pipe 8 has external copper fins attached to it to increase the rate of heat loss from the barrier fluid. The barrier fluid is recirculated back to the header tank by an outlet pipe 9, also finned, which joins the top of the header tank 7. The header tank 7 is mounted on a frame 11 which is rigidly connected to the bed plate 12.

The barrier fluid is circulated by means of a gear pump 10 attached to the inlet pipe 8 just below the header vessel 7. This positioning of the gear pump 10 ensures that the fluid in the header tank 7 can be maintained at a relatively low pressure as compared with fluid in the inlet pipe 8 below the gear pump 10. The gear pump 10 is driven by a timing belt or chain 13, one end of which passes around a timing pulley or sprocket 14 attached to the gear pump 10 and the other end or which passes around a timing pulley or sprocket 15 attached to the rotatable shaft 3 and positioned directly below the other pulley or sprocket 14.

In this way the rotation of the shaft 3 drives the timing belt or chain 13 which in turn rotates the timing pulley 14 attached to the gear pump 10 and drives the gear pump 10.

Appropriate choice of pulley or sprocket sizes can determine the rate of barrier fluid flow and its pressure for the given fixed rotational speed of the shaft 3.

Where the process pumping apparatus is installed on sites with potentially explosive chemicals it is vital that the barrier fluid should not run low thereby causing the seal 6 to overheat. To guard against this possibility the header tank 7 includes a probe 16 which controls the process pump motor 2 via an interlock. Should the probe 16 detect that the level of barrier fluid in the header tank 7 is low this will cut off the process pump motor 2 and prevent the seal 6 from running dry.

Claims (8)

CLAIMS:

1. A combination comprising: a double mechanical seal including a gland housing adapted to receive a rotatable shaft so as to define between them a cavity for barrier fluid; an inlet and outlet for barrier fluid communicating with the cavity; a pump for circulating the barrier fluid; and means for coupling the rotatable shaft to the barrier fluid pump so as to power the pump from the rotatable shaft.

2. A combination according to claim 1 further including: a header vessel for storing barrier fluid; an inlet pipe for directing barrier fluid from the header vessel to the double seal; an outlet pipe for recirculating barrier fluid back to the header vessel; and a pump attached to the inlet pipe adjacent to the header vessel.

3. A combination according to claim 2 in which the pump is a gear pump.

4. A combination according to claim 2 or claim 3 in which the header vessel is above the double mechanical seal.

5. A combination according to any one of claims 2-4 in which the pressure of barrier fluid inside the header tank is lower than that in the inlet pipe.

6. A combination according to any one of claims 2-5 in which the inlet and outlet pipes are finned.

7. A combination according to any one of claims 2-6 in which the gear pump is driven by a continuous flexible drive element, one end of which passes around a driven wheel which provides drive to the gear pump and the other end of which passes around a driving wheel which takes drive from the rotatable shaft.

8. A combination according to claim 7 in which the driving and driven wheel sizes are chosen to provide a desired rate of barrier fluid flow and/or pressure.