Building reliable, practically leak-free thermal fluid systems
Thermal fluid systems using synthetic organic and silicone-based heat transfer fluids are used in industries as diverse as pharmaceutical production, petrochemicals, and environmental test chambers. Regardless of the type of heat transfer fluid, application, or operating temperature, operators of these systems share one common desire: that the systems be as reliable and leak-free as possible (Fig. 1).
Reducing fluid leakage from a thermal fluid system has several benefits, the most obvious being economical. If the fluid stays in the system, new fluid does not have to be purchased to make up the losses. Leaking systems also affect production and increase maintenance costs.
In addition, there are hygienic benefits. Some thermal fluids have odors that plant personnel may find objectionable. Keeping leaks to a minimum, combined with adequate ventilation, contributes to a better workplace environment.
This article describes some best practices developed by The Dow Chemical Co. in the selection of piping, pumps, valves, flanges, gasketing, and expansion joints that are key elements essential to the design of a reliable, virtually leak-free system.
Dow acknowledges that other approaches have worked equally well for other thermal fluid users. Nevertheless, the practices presented here have been used successfully and were developed by Dow over many years as a user and manufacturer of heat transfer fluids. However, these practices should be considered as recommendations only.
The right connections
Most thermal fluid systems contain a number of flanged and screwed connections and each should be viewed as a potential leak point. For this reason, and for the sake of system operators who have to deal with leaks, a thermal fluid system should always be designed to include only as many breakable connections as are absolutely necessary.
Screwed connections should be avoided wherever possible. Bendable tubing, with compression fittings, has been found to be a satisfactory alternative. However, where screwed connections are desired, they should be no larger than 11/4, Schedule 80 pipe.
Tapered threads should be cut with a sharp, clean die. The threads should be washed with a solvent and pipe thread sealant applied. Under no circumstances should a pipe thread sealant be relied upon to make a good joint out of a poor one.
Flanged connections are by far the best compromise that can be made in terms of cost, versatility, and sealing ability. They are the most commonly used breakable connections in thermal fluid systems.
Several types of flanges can be used in thermal fluid systems. The choice of a flange is influenced by many factors, including:
Cost considerations, both initial and maintenance
Perceived performance differences in safety and emissions
Past operating experience
The ASME/ANSI B16.5 raised-face flange is the most prevalent. Typically, an ASME/ANSI Class 300 raised-face flange is chosen over an ASME/ANSI Class 150 raised-face flange. Since thermal fluid systems can operate as high as 750 F, a Class 150 flange frequently will not have sufficient strength for anticipated maximum operating pressures.
Some thermal fluid systems need to withstand rapid thermal cycling and the additional stresses induced by expansion and contraction forces. In general, a Class 300 flanged system is better able to maintain minimum required seating stresses during thermal cycling, resulting in a reduced risk of leaks.
Class 150 flanges can provide a suitably leak-free system at more moderate, relatively isothermal operating conditions. At 750 F, a carbon steel Class 150 flange has a maximum operating pressure of 95 psig. The vapor pressure of most heat transfer fluids exceeds 95 psig at this temperature.
To ensure proper gasket sealing, raised-face flanges must have the proper surface finish. Manufacturers of gaskets most commonly used to join ASME/ANSI B16.5 raised-face flanges will typically specify a phonographic surface finish of 125 to 250 microinch average roughness.
A gasket must be able to withstand the operating temperatures of the thermal fluid system and must be chemically compatible with the fluid used. Thermal fluid systems can operate from as low as -150 F to as high as 750 F. At these temperature extremes, all elastomers and plastics are unacceptable because of their poor mechanical properties.
Another temperature-related challenge is resistance to heat generated by an external fire. In the highly unlikely event that a fire occurs in the area of a flanged joint, the fluid should be kept contained within the system because thermal fluids do burn.
A gasket that can withstand extreme temperatures until the fire is extinguished can prove very beneficial in an emergency situation. Gaskets constructed with metal and graphite generally will have temperature and chemical compatibility as well as meet fire resistance requirements.
The physical design of a gasket must also be carefully considered. A catastrophic gasket failure, such as a blowout caused by unexpected over-pressurization of the system, will generate a large fluid leak, which in turn can lead to a potentially serious fire risk. Spiral-wound gaskets (Fig. 2) and grooved-metal gaskets can resist blowouts and are suitably constructed for leak-free thermal fluid system design.
Proper assembly of flanged joints reduces the chance of leaks. During installation, it is critical to protect the flange surface from nicks, dents, and scratches, all of which can cause the joint to leak.
Proper flange alignment is equally critical. Misalignment can result in overstressing one side of a flange while leaving the other side without sufficient compression to seal the gasket. Once a flange pair is properly aligned, the use of ASTM A-193 Grade B7 studs and bolts with ASTM A-194 Grade 2H hex nuts provides a fastener strong enough to supply the required sealing force.
One way to reduce leaks from valves would be to eliminate the stuffing box. This can be done by utilizing valves with a metal bellows as the primary seal in combination with high-temperature graphite packing as a secondary seal. However, the cost and space requirements of bellows-seal valves have limited their use in thermal fluid systems.
When choosing packed valves, pay particular attention to the specifications of the valve stem, stuffing box, and packing. In general, nonrotating, rising-stem valves are preferred to quarter-turn valves (Fig. 3).
For rising-stem valves, the suggested roughness of the stem-sealing surface should be a maximum of Ra 0.8
As with gasket materials, flexible graphite is the valve-stem packing material best suited for the entire operating range of most thermal fluid systems. One possible improvement to graphite-packed stem seals is to apply a live load to the packing follower (Fig. 4). This will ensure that as the packing wears, it will remain properly compressed against the valve stem.
Since valve stems are potential leak sources, they should be installed with the stem in a horizontal position if possible, provided the valve manufacturer does not advise against this orientation.
With the valve stem oriented horizontally, should a leak occur, the fluid will drip away from the valve rather than down into the insulation around the valve.
Globe, gate, and rising-stem ball valves are the preferred choices for thermal fluid systems. There are general specifications for these types of valves in 2-in. and larger sizes (see table). Specifications for smaller valves will vary from these.
One strong reason to design leaks out of a thermal fluid system is to reduce the risk of fire. It is well understood that a combustible fluid can ignite at temperatures well below its published autoignition temperature if spread out in a thin film. The high surface area present in many types of insulation can promote this phenomenon when soaked in a thermal fluid.
To minimize this risk, closed-cell insulation should be used in the immediate vicinity of the most likely leak points, such as valves and connectors. Fibrous insulation can be used for pipe runs between connectors and valves.
A properly installed drip ring ensures that any fluid getting under the insulation on one side of the ring will not migrate down the pipe, and end up soaking insulation on the other side.
While careful system design can go a long way to ensuring relatively leak-free performance, proper maintenance over the life of the system must not be ignored.
For example, while a system may last 20 years, valve packings will not. At some point, valves will have to be serviced. Proper system design and equipment selection, coupled with routine maintenance, helps keep the number, size, and frequency of leaks to an absolute minimum.
More Info: If you have any questions on thermal piping systems, call the Dow Chemical Co. Customer Information Group, 800-447-4369. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, email@example.com
Preferred valve specifications
Valve Gate Globe Rising stem ball Class API 600/Class 300 API 600/Class 300 API 600/Class 300 Flange Raised face Raised face Raised face Material Cast carbon steel ASTM A 216 Grade WCB Cast carbon steel ASTM A 216 Grade WCB Cast carbon steel ASTM A 216 Grade WCC Stem Rising Rising Rising OS&Y Yes Yes N/A Bonnet Bolted, full port type Bolted Bolted Bolting ASTM A 193 Grade B7 ASTM A 193 Grade B7 ASTM A 193 Grade B7 Seating Hard-faced, 13% chrome alloy steel trim, flexible wedge Hard-faced, 13% chrome alloy steel trim 316 SS seating, 316 SS seat inserts, nickel plated carbon steel ball Packing Flexible graphite Flexible graphite Flexible graphite Packing type Die-formed Die-formed Die-formed