Water is normally considered a good lubricant and can do an adequate job of providing lubrication between the lapped faces of a mechanical seal, but there are a few problems:
At elevated temperature the water lubricating film is not thick enough to separate the sliding surfaces of the seal faces. Cold water has a film thickness of about one micron which will keep lapped seal faces separated most of the time. Hot water has a film thickness of only one third to one half of that amount depending upon the temperature.
At some combination of temperature and pressure the water will vaporize, expand, and open up the lapped seal faces. When this occurs:
1.The carbon outside diameter can become chipped and damaged as the constant vaporizing and subsequent cooling vibrates the seal faces, causing them to bang together. Drive lugs will wear, metal bellows can break and lug driven hard or soft, faces can crack.
2.Solids dissolved or suspended in the water will be left between the seal faces when the water vaporizes. They'll imbed into the softer face, causing severe wear and damage to the hard face.
3.A phonograph finish can form on the carbon if a large particle of scale, or any foreign matter is blown across the two faces. The seal will leak through this damaged face.
'Slip stick' can occur because the faces are trying to stick together due to a lack of lubrication between them. The alternating sticking and slipping will produce a vibration that will chip carbon, break bellows and crack lug driven faces unless some form of vibration damping has been installed.
In many piping systems magnetite ( Fe304 ) forms on the inside surfaces as a corrosion resistant covering. This magnetite breaks loose from the piping walls and often collects on the seal components. It can be recognized by its black or reddish color and its attraction to a magnet. The magnetite affects the seal a couple of ways :
1.Being an abrasive material, it'll mechanically attack the seal sliding elastomer by penetrating into it. This will cause 'hang up' and eventual leakage.
2.It'll wear the sliding elastomer sealing surface.Loose magnetite is very common in new water systems. The problem will eventually clear its self up after the system has been in use for about a year and the ferric oxide has formed into a stable layer.
Hot water is dangerous. The leakage will be invisible as it flashes to steam. If the hot water is part of a condensate system it may have to be sealed under vacuum conditions.
In order to seal hot water effectively, you must address all five problems at the same time. We'll begin by learning how to pick the correct materials for the seal components, then we'll choose a seal design and finally apply the correct environmental controls to insure that the above problems are being addressed.
Picking the correct seal materials:
1.The seal face combination should be an unfilled carbon graphite running against either solid silicon carbide or tungsten carbide. Plated or coated faces should not be used in this application. A new face material made from graphite impregnated silicon carbide has become very popular in recent years because of its better heat conductivity.
2.The elastomer. Use ethylene propylene to 275 degrees Fahrenheit (135 C.) If you seal at a higher temperature, either Dupont's Kalrez or an equivalent will be necessary. In most cases you should be trying to cool the water to increase the face life. If the water is cooled a high temperature elastomer is not necessary. Be sure that you do not put petroleum grease on the ethylene propylene. Any petroleum product will attack ethylene propylene rubber (epr)
3.The metal components. 316 grade stainless steel is preferred. Metal bellows or springs should not be manufactured from stainless steel to avoid chloride stress corrosion problems. Hastelloy 'C' is your best choice for the springs or metal bellows.
Choosing the correct mechanical seal:
A balanced, o-rings 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. Don't 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 babbitt 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 some designs have trouble when the 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 mechanical seal face that can cause the lapped seal faces to become distorted.