Liquid vaporizes at mechanical seal faces because it loses pressure and gains friction heat as it leaks across them, and it is prevented by keeping seal chamber pressure far enough above the liquid’s vapor pressure, either by raising that pressure or by cooling the liquid. Where neither can reach enough margin, the answer is a different seal arrangement: one built to run on the vapor, or one that keeps the process liquid off the faces.
When the liquid film that cools and lubricates the faces flashes too early, the faces run partly dry. On a running pump that sounds like popping at the gland, often with puffs of vapor; on the bench it shows as chipped face edges.
Why the Film Flashes Below the Boiling Point
The film flashes because the liquid is below its boiling point only at seal chamber pressure, and as it leaks across the faces its pressure falls to atmospheric while face friction heats it. Somewhere on the way it reaches the boiling point for the pressure it is at.
On a contacting wet seal, some vaporization is designed in. Seal makers lay out the face geometry so the liquid turns to vapor about three-quarters of the way across the faces from the chamber side. Failure starts when the vapor forms closer to the chamber, as hotter liquid, lower chamber pressure or more face heat shrinks the liquid band.
Duties Prone to Vaporization
The duties prone to vaporization at the faces are light hydrocarbons and hot water, the services where the liquid in the seal chamber runs close to its vapor pressure.
- Light hydrocarbons: Propane, ethane and NGL mixes in refineries and pipelines.
- Hot water: Boiler feed pumps and other water services running at or near the atmospheric boiling point.
On these duties, a seal that ran well starts flashing early when the liquid is lighter or hotter than it was designed for, or when seal chamber pressure falls below the design value.
How Much Vapor Pressure Margin a Seal Needs
API 682 requires at least 3.5 bar (50 psi) of vapor pressure margin, the difference between seal chamber pressure and the liquid’s vapor pressure at chamber temperature. The Hydraulic Institute’s general rule is 175 kPa (25 psi). The two figures differ in scope: API’s is written for hazardous, flammable and toxic services, while HI’s is a rule of thumb that HI itself says still needs a check of the full application.
Where 3.5 bar cannot be reached, API 682 accepts a seal chamber pressure of at least 1.3 times the vapor pressure, a ratio that only works with both pressures in absolute terms. API 682 also requires the chamber to sit at least 0.35 bar (5 psi) above atmospheric. On large shafts, light liquids and high speeds, even 3.5 bar can be marginal.
Checking the margin on a pump needs the seal chamber’s own pressure and temperature. Where the chamber has no gauge, calculate its pressure from the pump’s suction and discharge pressures. Then look up the liquid’s vapor pressure at chamber temperature and compare the two on one basis, since vapor pressure tables give absolute pressure and gauges read gauge pressure.
Water at 140 °C has a vapor pressure of 3.6 bar(a). The 3.5 bar rule then needs 7.1 bar(a) in the chamber, about 6.1 bar on the gauge, and the 1.3 ratio needs 4.7 bar(a), about 3.7 bar on the gauge.
Raising Seal Chamber Pressure
Seal chamber pressure is raised by feeding the chamber from a higher-pressure source, pump discharge or an outside injection, and restricting the flow out with a close-clearance throat bushing.
- Plan 11 with a tighter throat bushing: A throat bushing on its own raises chamber pressure only slightly; a floating one cuts the clearance by more than half compared with a fixed bushing and is recommended for hot liquids with a high vapor pressure.
- Plan 14 on vertical pumps: Plan 14 holds flush flow and margin independent of the throat bushing below the chamber, and it vents the chamber as well.
- Plan 32 injection: A clean liquid injected at a pressure above the process and held in by a throat bushing suits liquids at or near their boiling point where other plans do not work. The injected liquid must stay liquid at pump temperature.
- Suction pressure set point: On pumps whose chamber follows suction pressure, such as NGL pipeline pumps, the minimum suction pressure set point has to include the seal’s margin.
Cooling the Liquid at the Faces
Cooling restores the margin by lowering the liquid’s vapor pressure at the faces, and the plan for it is Plan 23, in which a pumping ring in the seal circulates the chamber liquid through a cooler and back in a closed loop. It is the usual choice for water above about 80 °C, boiler feed pumps especially, and in hydrocarbon and chemical services at risk of flashing.
Plan 21, which runs discharge liquid once through a cooler into the chamber, is the fallback where a pump lacks the space Plan 23 needs for its pumping ring.
On either plan, keep the throat bushing, which stops hot pump liquid from mixing into the cooled liquid around the faces. A Plan 23 loop also has to be fully vented, since vapor trapped at a high point stops it circulating. On a Plan 21 handling a liquid near its vapor pressure, put its orifice after the cooler.
On light hydrocarbons, a metal-impregnated carbon carries face heat away better than a resin-impregnated one, and on smaller chemical pumps a tapered seal chamber with flow modifiers (Plan 03) keeps liquid circulating past the faces with no piping.
When No Margin Is Possible
When neither pressure nor cooling can keep the liquid in the gap, choose a seal that does not need it there. API 682 allows a non-contacting inner seal that lets the liquid vaporize across the faces and runs on the vapor like a gas seal; the other route is a pressurized dual seal, whose barrier fluid lubricates the faces in place of the process liquid.
The non-contacting inner seal is meant for liquids with a high or mixed vapor pressure where vapor suppression is not reliable. It leaks more than a contacting seal, so more vapor goes to the flare or vapor recovery system.
Pair it with a dry-running containment seal that collects the vapor leakage (Plan 76), not with a liquid buffer. A buffer liquid adds the outer seal’s heat to the inner seal and narrows a margin that is already thin, which is also why a contacting inner seal on Plan 52 is a weak choice for a flashing product.
The pressurized dual seal for this service is Arrangement 3, with a Plan 53 barrier held above seal chamber pressure. It also suits services where the product’s properties vary, as on a light-hydrocarbon pipeline carrying several liquids.
