Friday, July 29, 2016

Packing vs Mechanical Seal

Note: Cornell Pump Market Managers provide periodic articles to the blog, to discuss issues and developments and pump. The articles are meant to be more conversation and less technical, while still explaining important pumping concepts. In this edition, Cornell Pump Agricultural Market Manager Bob Jansen discusses differences between packing and mechanical seals.

Packing is the traditional method of stopping leakage around the drive shaft of an end-suction centrifugal pump. Rings of braided, fibrous material, such as graphited or non-graphited acrylic, PTFE (polytetrafluoroethylene or Teflon) or other materials, are “stuffed” into a pump stuffing box (or seal chamber), located in the pump backplate, around the outside diameter of the pump shaft, to reduce the high pressure developed in the pump case, and decrease the amount of pumped medium that is forced out of the pump along the drive shaft. Packing does not stop all leaking, however. Packing rings are kept just loose enough to allow a trickle of liquid to seep out during operation, which accomplishes the flushing action necessary to prevent overheating and excessive wear between the inside of the packing surface and the outside diameter of the drive shaft. 
Advantages of using packing normally include lower upfront cost, availability, simple installation and is replaceable with minimum down time. The disadvantages of packing are that it needs to be monitored and adjusted, as required, to maintain a slow drip (40-60 drops/minute) to cool and lubricate the area. In addition, the area surrounding the pump needs to be able to handle the small amount of constant and necessary leakage from the packed stuffing box – such as removing the liquid from the area, etc.
Packing is available from most manufacturers in pre-formed sets for each individual model, or in bulk form. Various materials are available to match the requirements of nearly every application.

Mechanical seals are another common method to seal this area of the pump. They consist, in their most basic form, of two flat faces (machined within light bands tolerance): a rotating element that spins with and seals the pump shaft, and a static element that presses into the backplate casting and seals the liquid from escaping to the atmosphere. The central advantage that mechanical seals possess over traditional packing is that they almost completely negate all forms of leaking in a pump. (A small amount of liquid vaporizes as it crosses the seal face, but is usually not noticeable). Less maintenance time is required with a mechanical seal during operation, as packing must be regularly inspected and adjusted/replaced, whereas a mechanical seal operates, without attention, until the seal faces are badly worn. Mechanical seals come in a number of possible configurations and materials for different pump types, medium pumped, pressure ranges required, etc., but for this post we will only mention the two overarching categories of single and double seals.
As with all mechanical seals, they require careful and informed installation, but once properly set in place, they require no additional adjustment or maintenance. Mechanical seals often present more upfront cost than packing – sometimes considerably more. However, because mechanical seals usually offer less downtime and maintenance than packing, they can save money over the life of the pump. Seals are also less tolerant to shaft deflection and misalignment, and dirty or contaminated medium.

Since mechanical seals require a thin liquid film to properly lubricate and cool the faces, especially with abrasive or corrosive pump mediums, the seal faces will be eroded over time. Proper materials must be selected to be compatible with the pumping medium, and can be expensive.

For hazardous or more severe pumpage, a double (dual pressurized) seal may be necessary. In this system, there is essentially no leakage allowed outside the pump. Double seals are far more durable than single seals and may last up to five times longer in this environment. A flushing liquid is required for double seal installations.
One method Cornell developed to assist with solids and abrasive applications is our patented Cycloseal® technology. Cornell’s “Run-Dry” System lubricates the mechanical seal and allows the pump to run without pumping liquid. Contact the factory for additional info on these sealing technologies.