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Peculiarities of mechanical seals for high-temperature pumps

Role of mechanical seals in a pump construction

One of the main characteristics of pump high operability is pressurization, especially while working with toxic, hazardous and radioactive fluids. Mediums pumped at high temperatures (up to 400°C), which are characteristic for oil refining procedures, can also be treated as hazardous. The defining role in pressurization belongs to a mechanical seal. At the mentioned working conditions double or “tandem” seals are to be used. Structurally, they can be of bellows or elastomeric types. Peculiarities of such types are given below, regarding the purpose of a seal construction.

Peculiarities of a seal construction

At early stages, secondary seals of stationary elements were usually made of resins with different properties. In the course of time the materials were being improved: their resistance against high temperatures and hazardous properties of a medium had been significantly increased. So, the scope of seal appliances was essentially widened.

Nowadays, as secondary seals, elastomeric gaskets are practically used. As a rule, they are O-rings installed between joined parts, in order to divide pumped and buffer mediums and environment itself. Being deformed, when between sealed surfaces, the gaskets provide for part movement, regarding each other, and reliable separation of mediums. Appliance of rings from various rubbers and polymers, or combined rings with fluoroplastic cover gives the opportunity to seal almost all range of liquid pumped mediums. Availability, low price, easy installation and maintenance are the basic advantages of elastomeric gaskets for mechanical seals of different types (single, double or “tandem” ones.)

Elastomeric gaskets can reliably operate without any special systems at medium temperatures of 200-250°C. To achieve reliable pressurization at 250-400°C (in oil refining, for example) had been a problem. Chief foreign and Russian manufacturers of mechanical seals solved it in different ways.

Foreign experience

The problem was clear and quite concrete. So long as the material of secondary seals wasn’t able to resist high temperatures, new types of materials were required. So, thermally expanded graphite materials, stainless steel and special alloys started to be used. Such companies as Burgmann, John Crane, Flowserve, etc. developed and implemented bellow seals.

Axially movable seal collar with elastomeric gaskets, which supports one of the rings of a friction pair, was replaced with a bellow block. The block represented a metal unit with one end welded to a metal ring with a pressurized antifriction ring, and the other end – to a metal sleeve for a sealing ring. Pressurization of joint parts was performed by means of thermally expanded graphite rings.

In order to make a right choice of seals and support systems, API (American Petroleum Institute) developed API 682 Standard which made choosing and designing of seals essentially easier. The standard also contains information concerning seal chambers of pumps.

Russian experience

In the times of USSR one of the leaders among mechanical seal producers for petrochemistry and refineries was Nalchik engineering plant. Seals designed and manufactured by the plant, showed satisfactory results while operating at 150-200°C. For temperatures of up to 400°C another means was used – cooling of a seal zone with a shaft heat-exchanger.

However, because of uncompetitive leakage characteristics, through antifriction pair, they didn’t get any further development and surrendered leadership to bellow seals by foreign manufacturers.

The situation changed when in 1992 ANOD had come to the market. ANOD’s specialists, who had been in the field of mechanical seals for nuclear industry since 1966 and in 1975 were awarded by USSR government, designed, manufactured and implemented seals with very low leakage rate. The cooler of the seals was performed as two-channeled, with good capability for heat exchange and strong turbulization of working mediums. Being installed into a pump seal chamber, between a shaft and chamber walls, the cooler limited heat supply to the seal out from a pressure casing and a shaft. In addition, the innovation was appreciated due to the fact that it provided for a reliable seal operation at 400°C without change of resin rings into thermally expanded graphite ones. Since 1996 implementing of mechanical seals with the cooler was started.

Operational features of high-temperature bellow seals.

Let’s observe operation of a pump unit furnished with a double bellow seal of a “tandem” type. Because of high temperature of a pumped medium, the heat is transferred into environment through the seal, chamber, shaft and buffer fluid that circulates in the seal. So, what factors do influence the seal and pump components?

Buffer fluid. Buffer fluid is separated from a pumped medium with the help of a bellow metal membrane of 0.2 mm thick. Being pressurized between a sleeve and bellow, buffer fluid starts evaporating and condensing in an impeller’s cavity or a channel. This results in bellow vibration, solid residue sedimentation on its internal surface and loss of buffer fluid’s functional characteristics. Intense heat exchange through a bellow leads to necessary increasing of heat withdrawal from the cooler of a buffer fluid reservoir and frequent fluid renewal.

Bearing assembly. Pumped medium heat that transfers along a shaft to a bearing assembly, does not meet any intense cooling from buffer fluid’s side, because it is screened by means of a sleeve and other seal elements. In order to escape heat transfer, increased cooling of bearings or choosing of a pump with a distant bearing assembly are required. Though, it can result in undesirable shaft extension and exceeded vibration level.

Bellow block. Acting like a spring and a pressurizer, a bellow block, when influenced by medium pressure, must stay flexible and provide for certain rigidity. However, it perceives significant loads due to cyclical fluctuations, as a result of unstable thermodynamic processes of a buffer fluid, and misalignment of a shaft axis relative to rotation plane of antifriction rings. Such requirements limit the use of one-layer bellows with 2.0 MPa pressure.

Shaft sealing. Sealing of a shaft is performed with the help of thermally expanded graphite rings (or of similar materials), being deformed in the sleeve by a thrust bushing, flanges and screws. Usage of such materials and the way of mounting are applied because of shaft high temperatures in the point of seal installation, and decreased elasticity of used materials.

Repair. The main reason for seal repair is pressurization loss of an antifriction pair. The distinctive feature of bellow seals is re-assembly or removal of the whole seal if any of the elements (antifriction rings, bellows or a secondary seal ring) gets out of order. Meanwhile it is important to install a stationary antifriction ring perpendicularly to the shaft axis. Usually, misalignment tolerance is not more than 0.5 mm. But this is hard to achieve while operating.

Operational features of high-temperature seals with a cooler

A shaft cooler is a constituent part of a mechanical seal and is installed in a seal chamber, in alignment with a shaft and a chamber. Structurally, a cooler is pressurized and has channels for a cooling liquid circulation. At the point of higher heat influence of a pumped medium (mechanical seal part of a cooler), a heat barrier is provided. Being installed with minimal clearances, a cooler provides for intense heat withdrawal from a shaft and a seal chamber. A seal flange, with cooling liquid inlet and outlet channels, also serves as a barrier that limits transfer of heat to the seal. As a result, at the point of elastomeric gasket installation, temperature doesn’t exceed 100°C.

Buffer fluid. Buffer fluid doesn’t have any high temperature influence, because the cooler reduces temperature of a pumped medium 3-4 times. That is why, when choose a buffer fluid, it is not required to take into account such factors as boiling, thermal decomposition and anticorrosion characteristics.

Bearing assembly. A cooler, installed in front of a seal, excludes heat transfer along the shaft and into the bearings. Meanwhile it is not required to increase power of a bearing cooling system.

Seal assembly. Seal assembly has antifriction rings with elastomeric gaskets. Pre-pressure of rings is performed by means cylindrical helical springs. All the parts do not demand for rigid coupling to each other and, if required, can be replaced.

Shaft sealing. Shaft sealing is performed by means of standard resin rings, installed into seal grooves without any auxiliary adjustment tools.
Repair. If any part of a seal (antifriction rings, elastomeric gaskets, springs) gets out of order, it can be replaced. Repair costs, time and volume of work are also reduced due to the fact that antifriction rings can be easily removed, grinded in and again installed.
So, the distinctive features of both seal types are shown in the table below:


 

Features

Bellow seal

Seal with an elastomeric gasket

1. Seal appliance

With high-temperature dissolvents

Provided

Limited by elastomeric gaskets resistivity

With high-temperature

compounds (hydrocarbons, etc.)

Provided

Provided

2.Replacement while operating on-site

Partially provided

Provided

3.Heat transferred to bearings

More

Less

4.Repair costs

60% of a seal price

20% of a seal price

5.Repair staff

high

standard

6.Quantity of secondary seals (rings)

Not less than 2

Not less than 4

7.Stability of buffer fluid characteristics

Low

High

8.Temperature in a seal body

Increased

Normal

9.Shaft heat transfer

Present

Absent

10.Shaft sealing construction

Complex

Simple

11.Access into a pump seal chamber

Provided

Hardly provided

 

So, the following conclusions can be made. Appliance of bellow seals is quite suitable in cases when repair costs are not a limiting factor: while pumping of dissolvents (if elastomeric gaskets may not be used) and in pumps with narrowed seal chambers.
In comparison with a thin-wall bellow seal, a seal furnished with a shaft cooler and elastomeric gaskets, provides more suitable conditions for antifriction pair operating: higher mechanical rigidity, lower medium temperature in the point of ring installation, better operability of fluoroplastic cover rings (or of composite materials).

Change of mechanical elastomeric seals into bellow ones is not always justified and requires a thorough analysis. This is confirmed by the leading companies – designers and producers of seals, as well as the customers. For instance, at Mariiski refinery bellow seals were changed into mechanical ones, with a cooler, and are successfully operating nowadays.

Manufacture analysis of the seals (for example, DN 60) for a particular enterprise, shows that manufacture costs of bellow seals are 20-25% higher, as compared to seals with a cooler. But because of high price of bellow units, the final price of bellow seals is 50-55% higher in comparison with mechanical ones. Bellow units by Chinese manufacturers reduce the price 6-7%. If a smaller diameter of bellows, price difference is also decreased.

Taking into account the information mentioned above and many years of experience in the field, one can come to the conclusion that as shaft seals for high-temperature pumps, cheaper seals with a cooler, as well as more expensive bellow seals, will be in demand.
So, while choosing the seals and support systems, it is preferable to consult with chief specialists and producers of the field who can advise a suitable seal type for particular operating conditions and a service mode.

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