Seal Application Section. How to seal liquids, gases and slurries SA017
If you’re thinking about converting a packed pump to a mechanical seal, you’ll have to decide if this is a logical seal application. SA016
What sort of life should you expect from a mechanical seal? S020
For an overview of the entire subject. SA001
There’s no end to the number of liquids, gases and slurries that have to be sealed. Many liquids are compounds manufactured from the mixing of different individual fluids, so we’ll learn how to put your products into sealing categories and then learn how to seal the. different categories . Here are the categories
- Fluids sensitive to small changes in temperature and/or pressure. SA007
- Fluids that require two mechanical seals. SA008
- Non lubricating liquids, gases and solids. SA009
- Slurries classified as solids in liquid. The solids may or may not be abrasive. SA10
- Liquids sensitive to agitation. SA011
- Liquids that react with each other to form a solid. SA012
- Lubricating liquids. SA013
There are some other categories of sealing that are independent of the liquid, gas or slurry. They are classified by their extreme operating condition and often require special seals:
- Hot products – Defined as too hot for one of the seal components, or hot enough to cause the fluid to change from a liquid to a gas or solid. Heat transfer oil is a good example of a fluid that will “coke” at elevated temperature.
- Cryogenic fluids – They present a problem for elastomers and some carbon faces. Liquid nitrogen or oxygen would be an example.
- High Pressure – Defined as stuffing box, (not discharge) pressure in excess of 400 psi. (28 bar). Pipe line and boiler-circulating pumps can have stuffing box pressures of this magnitude.
- Hard Vacuum – Defined as 10-2 Torr or below. This number is well below most condenser or evaporator applications, but does come up every once in a while.
- High Speed – Defined as the seal faces moving greater than 5000 feet per minute (fpm.) or 25 meters per second. Most process pumps do not approach this speed. The Sundstrand “Sundyne” pump is typical of a high-speed application.
- Excessive motion – defined as more than 0.005 inches (0,15 mm.) in a radial or axial direction. Mixers, agitators and specialized equipment have shaft movements up to 1/8 inch (3 mm). Long shaft vertical pumps and pumps equipped with sleeve or journal bearings, are another application for excessive motion.
- Excessive vibration – Unfortunately there are no reliable numbers for the vibration limits of mechanical seals. Most vibration studies have addressed only the bearings. It is important to know that excessive vibration can:
- Open the lapped seal faces.
- Chip the outside diameter of the carbon face.
- Break the metal bellows used in some seal designs.
- Wear the driving mechanism used to transmit torque from the set-screws to the seal faces.
- Loosen drive screws.
- Shorten bearing life
- Most seal designs can damage (frett) expensive sleeves and shafts.
- Some, but not all designs have built in vibration dampers to relieve some of these problems
You’ll need to select the type of seal you’ll be using in these applications.
- A balanced, O-ring mechanical seal should be used. Both rotating and stationary versions are acceptable although stationary is preferred. The O-ring will allow sealing in both directions if the application alternates between vacuum and pressure.
- A cartridge seal should be used for ease of installation and in the case of open impeller pumps, to allow for impeller adjustment as the pump cycles between operating and ambient temperature. Do not use cartridge mounted stationary seals unless they have been fitted with some type of self-aligning feature.
- A motion seal should be specified if the pump is equipped with sleeve or journal bearings. This is a very common arrangement with multiple stage boiler feed pumps.
- A high-pressure seal should be used if the seal chamber pressure (not the pump discharge pressure) exceeds 350 psi. (24 bar). High-pressure seals are of a more rugged construction that prevents face distortion and elastomer extrusion.
- Split seals can be used in some of these applications, but a few of the commercial designs have trouble when the stuffing box pressure alternates between a positive pressure and vacuum. Sleeve mounting the split seal helps with impeller adjustment, or in the case of vacuum applications the seal can be installed backwards, or with a discharge recirculation line installed to keep a positive pressure in the stuffing box. Note: many hot water applications are dangerous so dual seals are recommended.
- Care must be exercised if you use a stationary metal bellows seal design. Flow through the normal flush or recirculation connection can cause a substantial temperature differential across the seal face that can cause the lapped seal faces to become distorted.
One you’ve selected the seal, you’ll need to choose the seal materials:
- The metal components SA002
- The seal face combination.
- The elastomer (Rubber part) SA005
- Any elastomer you choose has a temperature limit SA005-1
Once you’ve installed the seal you should think about backup sealing. B006