Centrifugal Pumps Seals

CENTRIFUGAL PUMPS SEALS

Centrifugal Pumps Seals

Sealing The 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 Basics There 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 Basics Dynamic 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 Basics Pump 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 Basics Packing 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 Basics Packing 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 Basics Packing Seals:

Pumps designed to operate under suction lift conditions use a sealing or injectionWhy? type of packing arrangement. 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 Basics Packing 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 Basics Packing 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 Basics Packing Seals:

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

Why?

Centrifugal Pumps Seals

Sealing Basics Packing 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 Basics Packing 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 Basics Packing Seals:

Internally sealed

Centrifugal Pumps Seals

Sealing Basics Packing Seals:

Externally sealed

Centrifugal Pumps Seals

Sealing Basics Packing 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 Basics Packing Seals:

Centrifugal Pumps Seals

Sealing Basics Packing 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 Materials The 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 Materials In 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 Installation It 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 Installation To 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 Seals When 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 Seals 1. 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 seal faces. This feature compensates for seal face wear and wobble due to misalignment. Advantage: •It's inexpensive •Commercially available in a wide range of sizes and configurations.

Centrifugal Pumps Seals

Mechanical Seal Types : PUSHER: Incorporate secondary seals that move axially along a shaft or sleeve to maintain contact at the seal faces. This feature compensates for seal face wear and wobble due to misalignment. Disadvantage: •It's prone to secondary seal hangup and fretting of the shaft or sleeve.

Centrifugal Pumps Seals

Mechanical Seal Types : UNBALANCED: Advantage: •It's inexpensive. •Leak less. •More stable when subjected to vibration, misalignment, and cavitation. Disadvantage: •Its relative low pressure limit.

Centrifugal Pumps Seals

Mechanical Seal Types : CONVENTIONAL: Which require setting and alignment of the seal (single, double, tandem) on the shaft or sleeve of the pump. Although setting a mechanical seal is relatively simple.

Centrifugal Pumps Seals

Mechanical Seal Types : NON-PUSHER: The non-pusher or bellows seal does not have to move along the shaft or sleeve to maintain seal face contact. Advantage: •its ability to handle high and low temperature applications. •its does not require a secondary seal (not prone to secondary seal hang-up). Disadvantage: •Its thin bellows cross sections must be upgraded for use in corrosive environments.

Centrifugal Pumps Seals

Mechanical Seal Types : BALANCED: Balancing a mechanical seal involves a simple design change, which reduces the hydraulic forces acting to close the seal faces. Balanced seals have higher-pressure limits, lower seal face loading, and generate less heat. This makes them well suited to handle liquids with poor lubricity and high vapor pressures such as light hydrocarbons.

Centrifugal Pumps Seals

Mechanical Seal Types : BALANCED: Advantage: •Its higher-pressure limits . •Lower seal face loading . •Generate less heat.

Centrifugal Pumps Seals

Mechanical Seal Types : CARTRIDGE: Which have the mechanical seal premounted on a sleeve including the gland and fit directly over the special model shaft or shaft sleeve (available single, double, tandem). The major benefit, of course is no requirement for the usual seal setting measurements for their installation. Cartridge seals lower maintenance costs and reduce seal setting errors.

Centrifugal Pumps Seals

Mechanical Seal Arrangements : SINGLE INSIDE: This is the most common type of mechanical seal. These seals are easily modified to accommodate seal flush plans and can be balanced to withstand high seal environment pressures. Recommended for relatively clear non-corrosive and corrosive liquids with satisfactory' lubricating properties where cost of operation does not exceed that of a double seal.

Centrifugal Pumps Seals

Mechanical Seal Arrangements : SINGLE OUTSIDE: If an extremely corrosive liquid has good lubricating properties, an outside seal offers an economical alternative to the expensive metal required for an inside seal to resist corrosion. The disadvantage is that it is exposed outside of the pump which makes it vulnerable to damage from impact and hydraulic pressure works to open the seal faces so they have low pressure limits (balanced or unbalanced).

Centrifugal Pumps Seals

Mechanical Seal Arrangements : DOUBLE (DUAL PRESSURIZED): This arrangement is recommended for liquids that are not compatible with a single mechanical seal (i.e. liquids that are toxic, hazardous, have suspended abrasives, or corrosives which require costly materials). The advantages of the double seal are that it can have five times the life of a single seal in severe environments. Also, the metal inner seal parts are never exposed to the liquid product being pumped, so viscous, abrasive, or thermosetting liquids are easily sealed without a need for expensive metallurgy.

Centrifugal Pumps Seals

Mechanical Seal Arrangements : DOUBLE (DUAL PRESSURIZED): In addition, recent testing has shown that double seal life is virtually unaffected by process upset conditions during pump operation. A significant advantage of using a double seal over a single seal.

Centrifugal Pumps Seals

Mechanical Seal Arrangements : DOUBLE GAS BARRIER (PRESSURIZED DUAL GAS): Very similar to cartridge double seals ... sealing involves an inert gas, like nitrogen, to act as a surface lubricant and coolant in place of a liquid barrier system or external flush required with conventional or cartridge double seals. The gas barrier seal uses nitrogen or air as a harmless and inexpensive barrier fluid that helps prevent product emissions to the atmosphere and fully complies with emission regulations. The double gas barrier seal should be considered for use on toxic or hazardous liquids that are regulated or in situations where increased reliability is the required on an application.

Centrifugal Pumps Seals

Mechanical Seal Arrangements : TANDEM (DUAL UNPRESSURIZED): Due to health, safety, and environmental considerations, tandem seals have been used for products such as vinyl chloride, carbon monoxide, light hydrocarbons, and a wide range of other volatile, toxic, carcinogenic, or hazardous liquids. Tandem seals eliminate icing and freezing of light hydrocarbons and other liquids, which could fall below the atmospheric freezing point of water in air (32° F or 0° C). {Typical buffer liquids in these applications are ethylene glycol, methanol, and propanol.) A tandem also increases online reliability. If the primary seal fails, the outboard seal can take over and function until maintenance of the equipment can be scheduled.

Centrifugal Pumps Seals

Mechanical Seal Selection : The proper selection of a mechanical seal can be made only if the full operating conditions are known: • • • • •

Liquid. Pressure.

Temperature .

Characteristics of Liquid .

Reliability and Emission Concerns .

Centrifugal Pumps Seals

Seal Environment : STANDARD BORE STUFFING BOX COVER: Designed thirty years ago specifically for packing. Also accommodates mechanical seals (clamped seat outside seals and conventional double seals.)

Centrifugal Pumps Seals

Seal Environment : CONVENTIONAL LARGE BORE SEAL CHAMBER: Designed specifically for mechanical seals. Large bore provides Increased life of seals through improved lubrication and cooling of faces. Seal environment should be controlled through use of CPI or API flush plans. Often available with internal bypass to provide circulation of liquid to faces without using external flush. Ideal for conventional or cartridge single mechanical seals in conjunction with a flush and throat bushing in bottom of chamber.

Centrifugal Pumps Seals

Seal Environment : CONVENTIONAL LARGE BORE SEAL CHAMBER: Also excellent for conventional or cartridge double or tandem seals.

Centrifugal Pumps Seals

Seal Environment : LARGE BORE SEAL CHAMBERS: Enlarged bore seal chambers with increased radial clearance between the mechanical seal and seal chamber wall, provide better circulation of liquid to and from seal faces. Improved lubrication and heat removal (cooling) of seal faces extend seal life and lower maintenance costs.