Mechanical seal corrosion in a chemical pump is prevented by choosing every wetted part of the seal, from the O-rings to the springs, for the fluid at its actual concentration, temperature and impurity level. Where no material will survive that fluid, the seal needs an arrangement that keeps the fluid off its parts. In every duty, the temperature at the seal has to stay inside the range those materials can take.
Corrosion is one of several ways a pump seal fails, alongside dry running, installation errors and a poorly chosen flush plan.
Corrosion-Resistant Materials for Each Seal Part
Each seal part gets its own material, chosen for the fluid at its real concentration, temperature and impurity level, because the elastomers, faces and metal parts corrode in different ways. Two fluids with the same chemical name can attack a material at very different rates.
A chemical compatibility chart rates families of chemicals, so each rating has to be confirmed for the actual service. Trace compounds that nobody listed when the seal was specified are often what attacks it, so a complete chemical analysis of the fluid comes first.
Before installing, compare each choice with field experience in a similar duty. Re-check it whenever the process changes: a new concentration, a hotter operating point or a different cleaning chemical.
Elastomers
Elastomer attack is prevented by matching the elastomer family to the chemical family at the service temperature.
- FKM: the standard elastomer in most process-industry seals and API 682’s default. It is used widely in acids, water, lubricants and halogenated hydrocarbons, but it is not recommended for ketones, amines, low-molecular-weight ethers and esters, hot water or steam.
- EPDM: one of the best materials for hot water and steam, and also used with ketones and alcohols, so it is the alternative to FKM in hot water, steam and ketone services. Petroleum oils swell it until the seal fails, and that includes a petroleum-based lubricant used to assemble the seal or hydrocarbons that reach the fluid in a process upset.
- FFKM: the broadest chemical resistance of the elastomers. Dozens of compounds exist, and many give up resistance to some chemicals to gain it against others, so FFKM is specified by compound.
- PTFE and flexible graphite: chemically inert to almost any process fluid, but with almost no resilience. They replace elastomers in the most aggressive services and in batch processes where the chemicals change too often for any single elastomer grade.
FKM’s good rating for acids does not hold for every acid. In a published immersion test, an FKM compound held at 80 °C for 10 days lost about 80 % of its tensile strength in 20 % nitric acid.
Seal Faces
Seal faces corrode through the free metal or binder inside them, so the protection is to specify a face grade whose binder the fluid cannot attack.
Silicon carbide’s resistance depends on its grade. Caustics, amines, hydrofluoric acid and phosphoric acid carrying traces of HF attack the free silicon in reaction-bonded SiC and weaken the face. Sintered SiC contains no free silicon and resists almost any fluid a centrifugal pump handles, so it is the grade to specify in caustic service in any API 682 seal category.
Tungsten carbide for chemical duty is nickel-bound. Cobalt binder leaches out even in water and other mild chemicals. In nickel-bound grades, the free nickel sets the limit of their chemical resistance.
Most resin-impregnated carbons run in chemicals from strong bases to strong acids. Antimony-impregnated carbon rates poor in both acids and bases. For the most aggressive chemicals, acid-grade carbons are made from very pure carbon.
Alumina resists most process fluids only in its high-purity grades, and some strong acids attack even those.
Metal Parts
Seal metal parts start from 316 stainless steel and move to a nickel alloy chosen for the fluid when 316 is not enough.
A seal’s metal parts are much thinner than the pump’s, so a corrosion rate acceptable in a thick casing goes straight through them, and the seal’s alloy is chosen for the best resistance in the duty, even where the casing is a lesser metal.
316 handles many acids and caustics, but high chlorides pit it. The Nickel Institute’s guideline for continuous exposure at neutral pH and ambient temperature allows 316 up to 1,000 ppm chloride, less when hotter, and puts it at risk of sudden chloride stress corrosion cracking above 60 °C (140 °F). Those limits come from general water exposure, but a hot chloride duty past them calls for a better alloy.
Each upgrade alloy suits a different fluid:
- Alloy C-276: seawater, brine, many acids and strong oxidizers. It is used for sleeves, glands and fasteners when 316 will not do, and it is the standard spring alloy in many industries.
- Alloy 20: developed for hot sulfuric acid and still used there, normally only when no more common alloy is acceptable.
- Alloy 400: the usual seal alloy for hydrofluoric acid, for the metal parts and the welded bellows alike.
Keep the Chemical Away From the Seal
When no material will survive the fluid, keep it off the seal with a pressurized dual seal, or, on a single seal, with a clean external flush through a close-clearance throat bushing (Plan 32).
A Plan 32 flush injects clean liquid from an external source at a pressure slightly above the seal chamber. The flow has to be high enough to stop the corrosive fluid getting in, and the flush liquid dilutes the product. The seal’s materials still have to suit the process fluid at its worst, because that fluid reaches the seal whenever the flush flow drops.
A flush of the pumped fluid itself, such as Plan 11, brings the corrosive to the faces at full strength.
A dual seal keeps the fluid off the faces only when it is pressurized, as in API 682 Arrangement 3 with Plans 53A, 53B, 53C or 54. In an unpressurized Arrangement 2, such as Plan 52, the inner faces run in the process fluid.
API 682 recommends a barrier pressure at least 1.4 bar (20 psi) above the seal chamber, and the protection lasts only while that pressure is held at all times. Even then, the process side of the inboard seal stays wetted, so those parts need materials chosen for the process fluid.
The barrier fluid has to be compatible with the seal materials and with the process fluid it leaks into, and water is compatible with many aqueous process solutions.
Changing the seal type removes some wetted parts. A metal bellows seal does away with the dynamic O-ring, yet the API 682 Type B bellows seal has elastomer secondary seals; only the Type C, with flexible graphite secondary seals, has none.
For acids too aggressive for the elastomers and metals of a standard seal, a PTFE bellows seal keeps its springs and metal parts out of the fluid, leaving only the PTFE and the faces wetted.
Control the Temperature at the Seal
Hold the seal chamber inside a temperature range: cool enough to keep the seal materials within the limits they were chosen for, and warm enough to keep a crystallizing fluid in solution.
On a single seal in hot service, Plan 23 cools only the fluid circulating in the seal chamber, so its cooler carries a small fraction of the heat a Plan 21 cooler removes from the whole flush stream. For water and other aqueous solutions, most seals need the flush entering the chamber below 180 °F (82 °C). In a dual seal, the barrier fluid is typically kept below 150 °F (66 °C).
Bring cooling in gradually, because a cold flush reaching hot faces can crack a ceramic face through thermal shock.
Cooling also has a lower limit. A 50 % caustic soda solution begins to crystallize at 54 °F (12 °C), and stronger solutions solidify at much higher temperatures. Plan 23 is not used for fluids with high freeze points, and a barrier fluid that drops below its own freezing point stops circulating and fails the seal.
Where leakage crystallizes on the atmospheric side, a Plan 62 water quench keeps the crystals from forming.
