Classification of mechanical seals by arrangement
The mechanical seal acts as a check valve and a slider bearing. The obvious function is that of a check valve to prevent liquid under pressure from leaking out of the pump, or from drawing air into the pump when under vacuum conditions.Improper installation of mechanical seal can also is the major cause of failure. There are two types of mechanical seals:
Single seals installations
Single seals do not always meet the shaft sealing requirements of today’s pumps, due to the small amount of required leakage when handling toxic or hazardous liquids; suspended abrasives or corrosives in the pumpage getting between the seal faces and causing premature wear; and/or the potential for dry operation of the seal faces. To address these situations, the seal industry has developed configurations which incorporate two sets of sealing faces, with a clean barrier fluid injected between these two sets of seal faces. The decision to choose between a double or single seal comes down to the initial cost to purchase the seal vs. the cost of operation, maintenance and downtime caused by the seal, plus the environmental and user plant emission standards for leakage from the seal.Singl mechanical seals has following two subcategories:
The single inside-mounted seal is the most common in the industry and the most energy-efficient as compared with other sealing methods, such as packing and sealless equipment. They are used in all industries with respect to fluid types and their property ranges, pressure speed, diameter, and temperature.
Mounted outside of stuffing box.Only corrosion resistant materials are in contact with product.Hydraulically balanced.Simple design – Assembled at factory – no installation measurements required – Just slide on shaft and contact stationary seal ring tighten screws and remove setting clips.
Multiple seal installations
It is further divided into following two parts
Double seals consist of two single seals back to back, with the primary rings facing in opposite directions in the seal chamber. The neutral liquid, at a pressure higher than that of the liquid being pumped, lubricates the seal faces. The inboard seal keeps the liquid being pumped from entering the seal chamber. Both inboard and outboard seals prevent the loss of neutral lubricating liquid.The inner seal prevents the circulated fluid to enter the system and the outer seal prevents it from leaking it to atmosphere.The circulation of the fluid reuces the heat generated on the seal faces.This seals are available in different design combination suiting to service conditions.
- Single Acting
- Independent in the Direction of Rotation
- Internally Mounted
- Back to Back Design
A tandem mechanical seal arrangement is also a combination of two seals but they are mounted in the normal position, as opposed to the back-to- back position of a double seal. A barrier liquid is also injected between the two seals from an external reservoir in a closed loop piping system but it is not pressurized. The barrier fluid is circulated by the centrifugal force effect of a pumping ring in the seal. The pressure in the stuffing box is higher than in the barrier chamber so when the inner seal fails the process liquid enters the barrier chamber and forces the oil reservoir level to rise. A high oil level switch will trip an alarm and the pump is shut down. The process liquid is piped out from the of the reservoir into a safe disposal area.