Stop steam losses

Tag : Gate Valve Fittings
From water treatment through condensate return, leak-tight valvesand fittings reduce energy costs

Energy is the second largest component of cost in a manufacturingfacility, second only to raw materials or feedstock. It has beenreported that a large percentage of the fuel burned bymanufacturers is consumed to make steam. Steam leaks are a largecontributor to the cost of energy. This article looks at how steamleaks can occur and describes sealing methods that can helpminimize them.
Dave Simko
A steam system consists of a steam generator or boiler, adistribution system, point-of-use equipment, and condensate return.Steam is made in the generator by boiling properly treated waterusing an appropriate fuel. Once the steam enters the distributionsystem, it can lose energy in several ways.

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

Distribution and use

A distribution system transports the steam through a network ofpipes or tubes from the boiler to the point where it will be used.The objective of the distribution system is to deliver the steam toits point of use, losing as little energy as possible along theway. As soon as the steam leaves the boiler, it begins to transferits heat to any surface that is at a lower temperature. Thedistribution lines must be insulated to retain the heat. However,as the steam moves through the distribution system, some of itcondenses into water, which is at the same temperature as thesteam. Condensate build up in the system impairs the efficiency ofthe 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 mostdistribution systems.

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

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

Steam is a particularly difficult fluid to seal. It is veryerosive, so what may start out as a pinhole leak can rapidly becomea large plume of steam. It is not uncommon to see a screwedconnection 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 becausetheir large orifices and high flow coefficients (CV) minimizeenergy losses as the steam passes through them. However, valvestems seals can leak through a combination of wear, thermalcycling, and thermal shock. In applications where the valves arenormally closed (block valves), the same processes can also causeseat leaks and loss of steam.

The applications for steam are many and varied: cooking anythingfrom foodstuffs to wood chips, drying, treating fibers and othermaterials, and sterilization. All these applications require theirown pipework, fittings, valves and steam traps, which can be asource of leakage in the same way as distribution systems.

Tube fittings leak less

Screwed pipe fittings leak because the thread sealant used is notdoing its job, possibly because the wrong sealant was used, or itwas used and installed improperly, or it was not able to handlethermal cycling or extended use at higher temperature of thespecific application. The impact of extended use at hightemperature and thermal cycling has an even greater impact on valvestem and seat seals, especially in high-cycle applications, such asthose that can occur at the point of use.

A solution would be to install systems using tubing and two-ferruleflareless mechanical tube fittings. The use of tubing minimizes thenumber of fittings required in an installation and is particularlyeffective in close-coupled systems (Figure 1).

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

Live-loaded valves

As mentioned, ball valves are one of the most commonly used valvesin steam systems. A ball valve used in a saturated steam systemwill usually have a stem seal that takes the form of a ring of PTFEwith a square or rectangular cross-section, contained on the topand bottom with glands, and on the inside and outside diameters bythe stem and the packing bore respectively. The seal is made byloading the seal with enough force to deform the seal materialinward to make a seal against the stem and outward against thepacking bore.

During thermal cycling, the PTFE cold-flows into the peaks andvalleys on the machined surface of the stem. During mechanicalcycling, small amounts of PTFE are worn away. At a point, sometimesquite rapidly, the stem seal can become loose and steam leakagebegins. As mentioned earlier, steam is an erosive fluid, and whatbegins as a small seal leak can rapidly become a large leak.

A solution to consider is a ball valve with live-loaded stem sealsand seats. In the seal design shown in Figure 3, the seal member isa two-piece split chevron, rather than the conventional square orrectangular configuration. Although it is contained in the samemanner as the conventional seal, the loading is different. In thisdesign, the load is applied with a packing nut through conical discsprings located above the top gland. This provides a live-loadingforce, which compensates for wear during normal use, either atambient or elevated temperatures.

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

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