Centrifugal Pumps Seals

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CENTRIFUGAL PUMPS SEALS

Centrifugal Pumps Seals

SealingThe proper selection of a seal is critical to the success of every pump application. For maximum pump reliability, choices must be made between the type of seal and the seal environment. A sealless pump is an alternative, which would eliminate the need for a dynamic type seal entirely.

Centrifugal Pumps Seals

Sealing BasicsThere are two basic kinds of seals:Static seals :

Which are employed where no movement occurs at the Juncture to be sealed. Typical static seals are:Gaskets. O-rings .

Centrifugal Pumps Seals

Sealing BasicsDynamic seals :

Which are used where surfaces move relative to one another.where a rotating shaft transmits power through the wall of a tank. Through the casing of a pump or through the housing of other rotating equipment such as a filter or screen.

Centrifugal Pumps Seals

Sealing Basics

Centrifugal Pumps Seals

Sealing BasicsPump manufacturers use various design techniques to reduce the pressure of the product trying to escape such: 1. The addition of balance holes through the impeller to permit most of the pressure to escape into the suction side of the impeller. The addition of back pump-out vanes on the back side of the impeller.

As there is no way to eliminate this pressure completely, sealing devices are necessary to limit the escape of the product to the atmosphere. Such sealing devices are typically either compression packing or end-face mechanical seals.

Centrifugal Pumps Seals

Sealing BasicsPacking Seals:

The most common arrangement for gland packing used in pumps is the solid packed stuffing box It is important not to overtighten the packing gland so that all leakage is prevented, as it is this small amount of liquid, which lubricates the packing and so reduces friction and wear.

Centrifugal Pumps Seals

Sealing BasicsPacking Seals:

A replaceable sleeve is often fitted to the shaft in the region of the stuffing box. This simple type of packing arrangement should not be used when the pump is operating with suction lift conditions. Why? Because air may be drawn into the pump through the stuffing box. Causing the pump to lose suction. How to solve

this problem?

Centrifugal Pumps Seals

Sealing BasicsPacking Seals:

Pumps designed to operate under suction lift conditions use a sealing or injection type of packing arrangement. Why? This uses a liquid to help seal the packing gland and prevent air being drawn into the pump. This sealing liquid comes from either the discharge side of the pump or from an external source .

Centrifugal Pumps Seals

Sealing BasicsPacking Seals:

If the sealing liquid is supplied from the pump, then the stuffing box is said to be internally sealed. If the sealing liquid supplied from an outside source, then the stuffing box is said to be externally sealed. A sealing ring cage that often called a lantern ring is usually used to distributing the sealing liquid within the stuffing box.

Centrifugal Pumps Seals

Sealing BasicsPacking Seals:

Lantern rings are usually made of brass or bronze and are normally positioned mid-way in the stuffing box, with an equal number of packing rings at each side. If the pump is to handle liquids containing sand or grit, then the sealing system should be of the external type.

Centrifugal Pumps Seals

Sealing BasicsPacking Seals:

The pressure of the sealing liquid must be greater than the pump suction pressure.

Why?

Centrifugal Pumps Seals

Sealing BasicsPacking Seals:

A typical packed stuffing box arrangement consists of:Five rings of packing A lantern ring used for the injection of a lubricating and/or flushing liquid. A gland to hold the packing and maintain the desired compression for a proper seal

Centrifugal Pumps Seals

Sealing BasicsPacking Seals:

A flow from 40 to 60 drops per minute out of the stuffing box must be maintained for proper lubrication.

Centrifugal Pumps Seals

Sealing BasicsPacking Seals:

Internally sealed

Centrifugal Pumps Seals

Sealing BasicsPacking Seals:

Externally sealed

Centrifugal Pumps Seals

Sealing BasicsPacking Seals: When pumping slurries or abrasive liquids, it is necessary to inject a dean lubricating liquid from an external source into the lantern ring . A flow of from 0.2 to 0.5 gpm is desirable and a valve and flowmeter should be used for accurate control.

Externally sealed

Centrifugal Pumps Seals

Sealing BasicsPacking Seals:

Centrifugal Pumps Seals

Sealing BasicsPacking Seals: By a quench type gland. Water, oil, or other fluids can be injected into the gland to remove heat from the shaft, thus limiting heat transfer to the bearing frame. This permits the operating temperature of the pump to be higher than the limits of the bearing and lubricant design. The same quench gland can be used to prevent the escape of a toxic or volatile liquid into the air around the pump (smothering gland)

Centrifugal Pumps Seals

Packing MaterialsThe materials commonly used to make packing include cotton, asbestos and flax. They are usually woven or braided to form a continuous length of square section. The lengths of packing are often impregnated with graphite to assist in reducing friction and are available in a range of different section sizes to suit pumps having differing stuffing box dimensions. Packing lengths are often reinforced with wire strands which strengthens the material and helps it to retain its shape.

Centrifugal Pumps Seals

Packing MaterialsIn addition to the natural packing materials, there is also a range of synthetic and metallic packing materials, which may be more suitable for high temperature applications or for certain types of pumped liquid. Many of the synthetic packing materials are made in the form of a "V" or chevron section and are installed with the open part of the "V" facing the liquid being pumped. In this position, the pressure of the liquid in the pump tends to expand the packing and helps it to seat on the shaft.

Centrifugal Pumps Seals

Packing InstallationIt is important that the correct number of packing rings are used when inserting packing into the stuffing box. This can be achieved by one of the following: 1. By counting the number of rings previously removed. By checking the manufacturer's recommendations.

By measuring the depth of stuffing box (the width of the lantern ring must be deducted from the total depth of the stuffing box).

Centrifugal Pumps Seals

Packing Installation

Centrifugal Pumps Seals

Packing InstallationTo insert each ring of packing separately and push it into position with the gland. The rings should be dipped in oil before insertion (If the packing is not of the lubricated type). The location of lantern rings (if it is used). The thermal packing extension clearance. Checking the shaft or sleeve when re-packing a stuffing box.. A total re-pack job should be done whenever more than one ring of packing is required. It is normally permitted to add one extra ring of packing to a stuffing box that has a slight leak, but a note should be made when the ring is added so that more rings are not added at a later date.

Centrifugal Pumps Seals

Mechanical SealsWhen leakage of the pumped liquid is not acceptable, packing seals are not suitable and one of the many different types of mechanical seals is used instead. A mechanical seal is a sealing device, which forms a running seal between rotating and stationary parts. Advantages of mechanical seals over conventional packing are as follows: 1. Zero or limited leakage of product (meet emission regulations.) 1. Reduced friction and power loss. 1. Elimination of shaft or sleeve wear. 1. Reduced maintenance costs. 1. Ability to seal higher pressures and more corrosive environments.

Centrifugal Pumps Seals

Mechanical Seals1. The wide variety of designs allows use of mechanical seals in almost all pump applications.

Centrifugal Pumps Seals

The Basic Mechanical Seal

Centrifugal Pumps Seals

Principles of Operation: Stationary Seal Ring This is usually made of tungsten carbide or stainless steel and fits into the seal plate. Stationary Seal Ring Seal This is an O-ring which prevents leakage of liquid between the stationary seal ring and seal plate.

Centrifugal Pumps Seals

Principles of Operation: Rotating Seal Ring This is secured to the shaft and is pressed against the stationary seal ring by the combined action of the spring and the pressure of the liquid. The material used is normally carbon. Rotating Seal Ring Seal This is also an O-ring , which prevents leakage the rotating seal ring and the shaft.

Centrifugal Pumps Seals

Principles of Operation: Spring This is a single coil spring which helps to maintain contact pressure between the faces of the rotating and stationary seals.

Thrust Collar This is secured to the shaft by a grub screw and takes the reaction thrust of the spring. The position along the shaft where the thrust collar is mounted will control the amount of thrust exerted by the spring upon the sealing faces.

Centrifugal Pumps Seals

Principles of Operation: Seal Plate This seals the stuffing box and also houses the stationary seal ring. Seal Plate Seal This may be either an O-ring or a flat gasket which prevents leakage from the joint between stuffing box and seal plate.

Centrifugal Pumps Seals

Mechanical Seal Types :PUSHER: Incorporate secondary seals that move axially along a shaft or sleeve to maintain contact at the

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