Find the perfect steam trap

Consider a variety of factors to select a unit that comes the closest. What steam traps lack in size, they make up for in numbers. In a typical steam plant, their numbers range from several hundred in a large steam plant to more than 20,000 in refinery or chemical complexes.

By Stephen Frank, Richards Industries, Inc.

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The volume of condensate necessary to operate this trap is sensitive to freezing and the bucket can be damaged by water hammer. The inverted bucket trap has some versions that are light and compact, but it must only be mounted in the horizontal position. Main applications are process, drip leg and heating. Individual traps only handle a narrow range of pressures. No strainer or check valve is required, but a separate air vent or bypass may be necessary.

Disc

As condensate is fully discharged on startup, steam enters the trap and the flow velocity under the disc rapidly increases (Figure 3).

Figure 3. The disc steam trap is compact and light and often a throw-away trap.

Figure 3. The disc steam trap is compact and light and often a throw-away trap.

This reduces the pressure below the disc and pulls it down to close the trap. When the steam locked above the disc cools and condenses, it loses closing pressure and allows the cycle to repeat. The action is cyclic with some steam loss from each cycle.

The disc trap is compact and light, but horizontal mounting is preferred. Main applications are small process, drip leg and tracing. No check valve is required, but a strainer and a separate air vent may be necessary. Air can be trapped above the disc, locking the trap in the closed position. The use of a bimetallic ring to hold up the disc during start-up has been tried with limited success. Increased cycling can result when the trap is exposed to cold weather. Dirt can increase steam loss and disc wear, but freezing and water hammer aren’t problems.

Bellows

Alcohol/water mixture sealed into a bellows vaporizes as condensate approaches saturation temperature (Figure 4).

Figure 4. The bellows trap is a good choice for vertical orientations.

Figure 4. The bellows trap is a good choice for vertical orientations.

The resulting expansion of the bellows closes the valve. Vaporization temperature increases as operating pressure compresses the bellows, in a rough approximation to the steam curve. This results in a cyclic action that modulates from a minimum opening on very light load conditions to full opening at maximum load.

The bellows trap is compact and light and can be mounted in any position. Main applications are small process, tracing, radiators and general. No strainer or check valve is required. Freezing is no problem, but water hammer can fracture the bellows resulting in heavy losses of steam.

Capsule

Operation is the same as for bellows, but the alcohol/water mix is contained within a small, wafer-like element to reduce the motion and improve cycle life (Figure 5).

Figure 5. The capsule steam trap, a hybrid of the bellows, can serve in vertical installations.

Figure 5. The capsule steam trap, a hybrid of the bellows, can serve in vertical installations.

The action is cyclic, with steam loss each cycle from the blast discharge. Dirt in this short travel valve can cause steam loss.

No check valve is required, but a strainer may be necessary. The capsule trap is compact and light and can be mounted in any position. Main applications are tracing and drip leg. Freezing is no problem and with the “bellows” reduced to a single thin convolution, the risk of water hammer damage is greatly reduced.

Bimetallic-thermostatic

Temperature sensitive bimetal stack closes the valve (Figure 6).

Figure 6. The bimetallic-thermostatic trap is good for small, dirty applications.

Figure 6. The bimetallic-thermostatic trap is good for small, dirty applications.

This trap is individually adjusted to close at a fixed temperature, making it restricted to a narrow operating pressure range unless re-adjusted.  The action is modulating, with good cold capacity and fair hot running capacity. This trap is steam tight at set pressure only, blowing steam to low pressure startup or reduced-pressure operation.

A check valve and strainer may be necessary. This type of trap is compact and can be mounted in any position. The main application is tracing. Freezing is no problem, but dirt and wear in the bimetal stack cause erratic response and changes in performance.

Bimetallic-thermostatic/thermodynamic

A third generation design, the operation is the same as bimetallic-thermostatic, except a single delta-shaped element is used (Figure 7).

Figure 7. The bimetallic-thermostatic/thermodynamic trap is good choice for compact, difficult-to-classify applications.

Figure 7. The bimetallic-thermostatic/thermodynamic trap is good choice for compact, difficult-to-classify applications.

This design provides additional thermodynamic flow forces, developed in the downstream expansion chamber, to improve the hot discharge capacity right up to saturation temperature.

No strainer or check valve is required. This kind of trap is compact and can be mounted in any position. The main applications are drip leg, process, heating, tracing and general. Freezing is no problem and the single piece, bimetallic stainless steel element isn’t affected by dirt or wear.

Thermostatic type traps are often considered a universal steam trap, but no single type of trap is ideal for all applications. Ignoring the variety of choices available to simplify this decision can increase the true cost of the steam trap. Steam trap manufacturers can give more detailed information on the operating characteristics and total cost of each type. It’s important to remember that the price of a steam trap includes the cost of installation, maintenance and wasted energy during its operating life.

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