Stop steam losses
From water treatment through condensate return, leak-tight valves and fittings reduce energy costs

Energy is the second largest component of cost in a manufacturing facility, second only to raw materials or feedstock. It has been reported that a large percentage of the fuel burned by manufacturers is consumed to make steam. Steam leaks are a large contributor to the cost of energy. This article looks at how steam leaks can occur and describes sealing methods that can help minimize them.
Dave Simko
A steam system consists of a steam generator or boiler, a distribution system, point-of-use equipment, and condensate return. Steam is made in the generator by boiling properly treated water using an appropriate fuel. Once the steam enters the distribution system, it can lose energy in several ways.

If the pipework and fittings do not have adequate thermal insulation, excessive amounts of energy will be lost as heat, while the resulting condensation can also contribute to energy loss. Other energy losses happen as a result of steam leakage in steam traps, piping joints, and valve packings. Whatever the cause of the energy loss, the wasted heat must be made up in the steam generator, which means extra costs for water, water treatment, and fuel.

Distribution and use

A distribution system transports the steam through a network of pipes or tubes from the boiler to the point where it will be used. The objective of the distribution system is to deliver the steam to its point of use, losing as little energy as possible along the way. As soon as the steam leaves the boiler, it begins to transfer its heat to any surface that is at a lower temperature. The distribution lines must be insulated to retain the heat. However, as the steam moves through the distribution system, some of it condenses into water, which is at the same temperature as the steam. Condensate build up in the system impairs the efficiency of the system. Steam traps drain the condensate out of the system. However, they are dynamic devices and they do wear out and fail. Failed steam traps are a major source of energy loss in most distribution systems.

Another major source of energy loss in steam distribution systems is through leaks from valves and fittings. Connections in the larger size pipes in the system—the steam mains—are usually made by welding or with flanges. Smaller size pipe is used
to transport the steam from the take-off point on a steam main to the point of use. It has been estimated that as much as 80 percent of the smaller piping in a steam distribution system is 50 mm diameter or smaller. This part of the distribution system is constructed using a combination of welded connections for long runs and screwed pipe for the shorter, close-coupled sections.

Screwed pipe connections can be susceptible to steam leaks. The connection is made by screwing a tapered male NPT pipe end into a tapered female NPT fitting. A seal is made on the flanks of the pipe threads. The roots and crests are truncated (shortened), so the seal is not complete and a spiral leak path exists in the completed connection. Pipe dope, PTFE tape, or some other pipe thread sealant is used to fill this void during assembly of the connection and is the only barrier to leakage.

Steam is a particularly difficult fluid to seal. It is very erosive, so what may start out as a pinhole leak can rapidly become a large plume of steam. It is not uncommon to see a screwed connection leaking around its full circumference.

A similar situation exists with the valves used in steam systems. Gate valves and ball valves are commonly used for steam because their large orifices and high flow coefficients (CV) minimize energy losses as the steam passes through them. However, valve stems seals can leak through a combination of wear, thermal cycling, and thermal shock. In applications where the valves are normally closed (block valves), the same processes can also cause seat leaks and loss of steam.

The applications for steam are many and varied: cooking anything from foodstuffs to wood chips, drying, treating fibers and other materials, and sterilization. All these applications require their own pipework, fittings, valves and steam traps, which can be a source of leakage in the same way as distribution systems.

Tube fittings leak less

Screwed pipe fittings leak because the thread sealant used is not doing its job, possibly because the wrong sealant was used, or it was used and installed improperly, or it was not able to handle thermal cycling or extended use at higher temperature of the specific application. The impact of extended use at high temperature and thermal cycling has an even greater impact on valve stem and seat seals, especially in high-cycle applications, such as those that can occur at the point of use.

A solution would be to install systems using tubing and two-ferrule flareless mechanical tube fittings. The use of tubing minimizes the number of fittings required in an installation and is particularly effective in close-coupled systems (Figure 1).

The two-ferrule flareless fitting is an all-metal fitting that requires no additional sealing compounds to make a leak-tight seal (Figure 2). During make-up, the front ferrule seals on the outside diameter of the tubing, while the rear ferrule provides the holding power to maintain the connection to the burst strength of the tubing. During thermal cycling, the fitting compensates for the expansions and contractions that occur in the tubing and the connection as the temperature cycles between ambient and the full temperature of the steam.

Live-loaded valves

As mentioned, ball valves are one of the most commonly used valves in steam systems. A ball valve used in a saturated steam system will usually have a stem seal that takes the form of a ring of PTFE with a square or rectangular cross-section, contained on the top and bottom with glands, and on the inside and outside diameters by the stem and the packing bore respectively. The seal is made by loading the seal with enough force to deform the seal material inward to make a seal against the stem and outward against the packing bore.

During thermal cycling, the PTFE cold-flows into the peaks and valleys on the machined surface of the stem. During mechanical cycling, small amounts of PTFE are worn away. At a point, sometimes quite rapidly, the stem seal can become loose and steam leakage begins. As mentioned earlier, steam is an erosive fluid, and what begins as a small seal leak can rapidly become a large leak.

A solution to consider is a ball valve with live-loaded stem seals and seats. In the seal design shown in Figure 3, the seal member is a two-piece split chevron, rather than the conventional square or rectangular configuration. Although it is contained in the same manner as the conventional seal, the loading is different. In this design, the load is applied with a packing nut through conical disc springs located above the top gland. This provides a live-loading force, which compensates for wear during normal use, either at ambient or elevated temperatures.

The conical disc springs continually “retighten” the seal, ensuring its integrity and reliability over a longer service life. The conical disc springs also compensate for expansion and contraction during thermal cycling, maintaining the seal during the excursions. Live-loaded seats (Figure 3) ensure similar leak-tight results regarding seat leakage in applications, such as where the ball valve is used as a block valve to atmosphere and seat leakage would permit energy in the form of steam to leave the system.

Energy is a very large component in the overall cost of operating any manufacturing operation. Around 45 percent of that energy is in the form of steam. Steam leaks allow energy to leave the system. The energy is not recoverable and must be replaced, increasing the overall cost of the steam. Flareless mechanical tube fittings and ball valves with live-loaded stem seals and seats are a way to substantially reduce steam leaks and contribute significantly to cost savings in the overall operation of a steam system. n