Mechanical seal

inappropriate operation Mechanical seal Volcano

Unlike gland packings, mechanical seals have a sealing gap which is positioned at a right angle to the shaft axis. These shaft seal designs are also called axial or hydrodynamic mechanical seals Volcano. Compared with gland packings, they require less space and no maintenance.

Mechanical seals are well-suited for sealing low and high pressures and circumferential speeds. The risk of inappropriate operation is therefore very low.

considerable disadvantages Seal Volcano

However, considerable disadvantages arise through wear caused by abrasive fluids . As is the case with gland packings, clean barrier or flushing fluids (e. g. cleaned by means of cyclone separators) help to keep abrasive particles away from vulnerable seal faces.

differentiating feature Seal Volcano

Pressed together by hydraulic and mechanical forces, two seal faces slide relative to each other during operation. The sealing gap lies between these precisely machined seal faces and is filled with a lubricating film, generally a liquid. The sealing gap width (i.e. the distance between both seal faces) is influenced by various factors, including the seal faces’ surface quality (i.e. how rough or smooth they are) and the sliding velocity.

Leakage from mechanical seals Volcano is very low; the fluid leaks into the atmosphere in the form of vapour or droplets. To calculate the mechanical seal’s leakage rate, a gap width of under 1 μm is normally assumed. Thanks to this extremely narrow gap, the leakage rate for mechanical seals is considerably lower than that for shaft seals with radial gaps.

A further important differentiating feature is that seals can be unbalanced and balanced. In the case of unbalanced mechanical seals, the seal face is exposed to the complete pressure to be sealed.

In the case of balanced seal types, a shoulder on the shaft or shaft sleeve ensures that only a portion of the fluid pressure is active as an axial force.

Alongside the hydraulic closing force, spring forces provide an additional axial force acting on the sealing gap. The springs can employ an open or enclosed design and be in contact with the fluid handled or not; they may or may not transmit torque.

Spring types employed

Central spring, conical or cylindrical, mounted onto shaft as single spring
Multi-spring arrangement consisting of concentrically arranged multiple springs
Metal bellows
Wave springs

The friction losses generated are lower than those of gland packings. Heat is generated in the shaft seal housing due to friction; depending on the amount produced, it can be dissipated either via convection from the seal housing to the atmosphere or via forced circulation through an externally installed heat exchanger.

Frequently used designs

Single, unbalanced mechanical seal as a typical example for a centrally arranged, conical single spring: The variant shown here is for “dead end” installation, i.e. there is no additional fluid circulation in the mechanical seal area.

Unbalanced mechanical seals are used for pressures of up to max. 15 bar and sliding velocities of up to max. 15 m/s. In general, a sufficient proportion of friction heat generated in the sealing gap can be transferred to the fluid handled and dissipated from the shaft seal housing to the atmosphere via convection. If the fluid handled is cold, the friction heat is absorbed by the fluid itself. One variant is the rubber bellows seal (bellows-type mechanical seal).

Unbalanced mechanical seal with stationary spring assembly: this design is used for higher sliding velocities and ensures the springs can reliably fulfil their task (rotary spring assembly would entail a risk of broken springs due to high centrifugal forces).

Mechanical seal arrangements

Single seal arrangement
Multiple seal arrangement

In the case of “back-to-back” arrangements, a barrier fluid is fed into the space between the two mechanical seals. Its pressure should be approx. 10 %, and at least 2-3 bar, higher than the pressure of the fluid handled by the pump.
This barrier fluid ensures that the fluid handled does not leak into the atmosphere. Before considering this arrangement, it should be established whether a zero-leakage pump such as a canned motor or mag-drive pump would be more suitable for the application.

As the barrier fluid absorbs the friction heat generated by the two mechanical seals, it must be circulated, i.e. removed from the seal cavity, cooled and returned to the seals.

Canned motor

The canned motor is a special type of wet rotor motor, in which the stators winding is protected from the fluid handled by a cylindrical, particularly thin tube (e.g. made from stainless steel or plastic) inserted in the machine’s “air gap” (in the gap between stator and rotor). The can must have low or negligible electrical conductivity to avoid insulation and corrosion problems in the stator winding.

Mag-drive pump

Mag-drive pumps are seal-less pumps whose shaft torque is transmitted via a magnetic coupling drive with permanent magnets by means of magnetic induction.

To prevent any leakage to atmosphere through a containment shroud failure during operation, mag-drive pumps are sometimes designed with double-walled containment shrouds and a leakage monitor. See

Heat dissipation in the stator winding is complicated, however, because efficiency is reduced as a result of the eddy current losses in the can, the increase in the gap between the rotor and stator, and higher friction losses due to fluid friction at the rotor.

The power range of canned motors lies between several watts and approximately 2000 kW. Canned motors are employed wherever hot, aggressive, explosive, toxic or radioactive fluids need to be pumped, which is why a great deal of focus is placed on hermetically sealed pump sets without gland packings or mechanical seals. Canned motors are used for circulators reactor circulating pumps , and process pumps for chemical and process engineering applications.

The barrier fluid pressure is generated by a barrier fluid system (thermosyphon vessel) or pressure booster. In the case of tandem seals, the space between the seals is flushed by unpressurised quench liquid (quench). If the leaking fluid handled by the pump has a tendency to crystallise when in contact with air, a seal arrangement comprising two rubber bellows seals should be used. It is important that the quench liquid and fluid handled are compatible.

what is Quench?

Quench is the term used for an arrangement that applies an unpressurised external fluid to a mechanical seal’s outboard faces. The quench liquid can assume the following tasks:

Absorption/removal of the seal’s leakage
Monitoring of the mechanical seal’s leakage rate
Lubrication/cooling of the stand-by seal
Exclusion of air: prevents the leakage from reacting with the atmosphere (e. g. crystallisation of the leaking fluid)
Dry running protection

The quench fluid is sealed via throttling bushes, preliminary gland packings, radial shaft seal rings, lip seals or mechanical seals

Instead of using an outboard mechanical seal, it is also possible to install a simple sealing element such as a lip seal or packing ring. It is fitted as a back-up seal for the main seal to prevent leakage (e. g. in the case of hazardous fluids) and to safely and reliably dissipate heat.

Tandem seals are employed when a high internal pump pressure requires distribution to two mechanical seals. The barrier fluid pressure level then lies between the pressure to be sealed and the atmospheric pressure. The pressure handled by the inboard seal corresponds to the difference between the pressure to be sealed and the barrier fluid pressure; the pressure handled by the outboard seal corresponds to the difference between the barrier fluid pressure and the atmospheric pressure. The barrier fluid must circulate in order to dissipate the friction heat generated by the seals.

Non-contact seal with radial gap

This category encompasses all throttling gaps with or without labyrinths, pumping rings/screws and floating ring seals.

The sealing gap width between the stationary component and the rotating component is designed to be as narrow as possible in order to minimise leakage. However, it is important to ensure that the parts do no rub against each other. Leakage on a rotating shaft is slightly lower than during standstill.

The fluid flowing through the gap allows the pressure to be reduced in relation to the atmospheric pressure. On throttling gaps and floating ring seals this is achieved in the gap due to fluid friction and due to flow losses when the fluid enters or leaves the gap.

Floating ring seal

The major advantage of floating ring seals is the fact that the components are not in contact. However, the time and costs required to provide the barrier condensate, its treatment and relevant control equipment are substantial.
Thanks to their non-contact nature, these seals can be used for high circumferential speeds and mid-level pressures (30 to 50 bar).
With regard to their reliability, they are almost independent of the feed water’s chemical composition.
The floating ring seal consists of several short throttling rings fitted in succession which can move in a radial direction and centre themselves automatically due to the pressure distribution on the ring. A cold barrier condensate injected into the seal ensures that hot water from the pump does not escape to the atmosphere (controlled system). As long as the pump is in operation or under pressure, barrier water supply must be ensured.
The floating ring seal is occasionally employed in boiler feed pumps. Its barrier condensate quantity can be controlled via the barrier condensate’s pressure and temperature difference.
For differential temperature control
(Δt control), the difference between the temperature of the barrier condensate at the outlet and that of the injection condensate is defined. In the case of boiler feed pumps, the amount of feed water escaping from the inside of the pump is very low, while penetration of cold water into the pump can be ruled out.
For differential pressure control (Δp control), the difference between the injection pressure and the inlet pressure is defined. A very small amount of barrier condensate flows into the pump. This puts high demands on the cleanliness and gas-free condition of the barrier condensate required to prevent the main circuit from being contaminated.
Instead of a floating ring seal, a labyrinth seal can also be used.

Labyrinth seal Volcano

The labyrinth seal is a firm throttling bush with a circular groove profile. As radial movement is impossible with this type of seal Volcano, the diametral clearance must be wider than that employed with floating ring types. As a consequence, the leakage rate is higher, in turn requiring a higher barrier water flow

Centrifugal seal

This type of shaft seal generates pressure itself in order to counteract the differential pressure to be sealed; it is frequently backed up by a standstill seal. Designed as
spring-loaded mechanical seal, it is opened by centrifugal forces at very low speeds and thus protected against wear. See Fig. 20 Shaft seal
The actual centrifugal seal (fitted as an auxiliary impeller using a liquid ring at the outer diameter) operates contact- and wear-free.

Pumping ring/screw

Optimally designed pumping rings/screws (thread pitch of the stationary part directed against the pitch of the rotating part) can also generate a back pressure capable of balancing the pump’s internal pressure when the pump is running. The pressure balance achieved this way depends on the rotational speed, thread length, gap width and mean gap diameter.
Heads of 10 to 30 m can be achieved.
As soon as the shaft stands still, however, the pumping ring/screw has only a throttling effect comparable with a labyrinth gap.
If a pumping ring/screw is to serve as a pump seal, it needs to be backed up by a contact-type standstill seal.

Hydrostatic seal

Due its design, proper functioning of the hydrostatic seal as a non-contact seal is only ensured at pressures from 20 bar. The pump drive must not be started until this pressure level is reached.
As the seal is very sensitive to solid particles, the barrier water which feeds the seal must be extremely clean.
The sealing gap is self-adjusting. Depending on gap geometry and the pressure to be sealed, the sealing gap will adjust itself at about 10 μm.
The gap’s stiffness is very high at full operating pressure (160 bar). To move the gap from its balanced position by 1 μm, an external force of approx. 4000 N would be
necessary.
The gap with which the seal operates may be very narrow, but it is finite, and as such exhibits a considerable leakage rate (p = 160 = 160 bar, n = 1.500 = 1500 rpm; sealing diameter at 260 mm, Q = 800 = 800 l/h). It is therefore necessary to back up the hydrostatic seal with a low-pressure seal that provides sealing to atmosphere.

Hydrostatic-seal

Static contact seal

Static contact seals include O-rings. These are moulded seals and are defined as “rings with circular cross section made of elastic materials; they seal through the effect of slight bracing during installation, intensified by the operating pressure” according to DIN 3750. Their symmetrical cross-section rules out incorrect installation.

As connecting components can be easily calculated and designed, their use is widespread.

O-rings are employed on all the shaft seals described here. However, they can only be used as static sealing elements or to seal areas where slight axial movement is occasionally required.

They are manufactured at different hardness degrees, specified as shore hardness (A or D). The hardness scale ranges from 0 to 100, 0 being the lowest and 100 the highest hardness unit.

The majority of O-rings used on mechanical seals are elastomer rings with a shore A hardness of 70 to 90. These O-rings are used for sealing between the shaft sleeves and the shaft, and between the primary ring or the mating ring and the respective components they are connected with. They ensure that the spring-loaded seal component can follow small axial shaft movements.

Their significance is often underestimated: ultimately, each shaft seal is only as good as its O-ring. O-rings must be matched to the fluid handled, cover a defined temperature range and provide good ageing resistance. Moreover, it is important to use a high-quality O-ring grease which meets the operating requirements. Besides providing long-term lubrication, the grease must be compatible with the fluid handled and must not attack the O-ring.

Elastomers which swell less than 10 % in the operating fluid and do not chemically react with the fluid handled are suitable for use as a mechanical seal’s secondary seal. A number of elastomers are available for this purpose which react differently in a reference oil with regard to temperature
resistance or swelling properties.