Thermostatic Steam Trap

What is a thermostatic steam trap? 

A thermostatic steam trap is a steam trap that works by employing the concept of the temperature difference between steam and cool condensate and air. In this type of trap, steam increases pressure in a thermostatic element. This causes the thermostatic steam trap to close. When the non-condensable and condensate gases in the cooling leg back up, the temperature starts reducing and the thermostatic element contacts to open a valve. The quantity of backed-up condensate ahead of the thermostatic steam trap depends on steam pressure, load conditions, and pipe size. A thermostatic steam trap is also used to pass air from a system of steam. As the air collects, the temperature reduces and an air vent discharges air automatically at moderately below steam temperatures all-round the whole operating pressure range. Thermostatic steam traps can be available in either wafer-type elements or balanced pressure bellows. These steam traps are manufactured using a variety of materials such as carbon steel, stainless steel, and bronze. Thermostatic steam traps are used to handle light condensate loads. 

Figure: Thermostatic steam trap.

How does a thermostatic steam trap work?

A thermostatic steam trap can operate using balanced pressure bellows. In this case, the trap starts working by first pushing air and condensate ahead of the steam via the trap. The thermostatic element is wholly contracted and at the same time, the valve is kept open to a point when steam approaches the trap. When the temperature in the thermostatic steam trap increases, it immediately heats the charged element which then increases the inside vapor pressure. As the pressure in the element is balanced to the trap body system pressure, the element expands and it closes the valve. When the inside temperature of the trap reduces for few degrees lower than the saturated steam temperature, the element bellows is contracted by the imbalanced pressure which opens the valve. 

Working of a thermostatic steam trap using balanced pressure bellows

The other method of operation of a thermostatic steam trap is that of a balanced pressure wafer operation. This method is similar to that of the balanced pressure bellows. The wafer is partly filled up with a liquid. When the temperature in the trap increases it heats the charged wafer and thus increases the inside vapor pressure. As the pressure in the wafer becomes more than the surrounding steam pressure, the membrane of the wafer is pushed on the seat of the valve closing the valve. Drop-in temperature emanating from condensate or non-condensable gas cools and lowers the wafer inside pressure which allows the wafer to unveil the seat. 

Liquid expansion thermostatic steam trap has a fixed temperature discharge feature, this trap can be used as a shutdown drain thermostatic trap. In this case, the outlet always points upwards as shown in the figure below. This helps to enhance the continuous immersion of the element with oil. Since this trap can discharge at temperatures between 60 ^oC to 100 ^oC it can only open normally during start-up. The liquid expansion thermostatic steam trap installation can be done alongside the mains trap which is normally piped to a return line of the condensate. 

Illustration of liquid expansion thermostatic steam trap in use

Balanced pressure thermostatic steam trap 

The balanced pressure thermostatic steam trap is an improvement of the liquid expansion thermostatic steam trap. The working temperature of this trap is impacted by the steam pressure of the surrounding. This trap operates by the use of an element which is a capsule with a mixture of water and special liquid with a boiling point lower than that of water. At start-up, cold conditions exist and the capsule is relaxed. This trap has its valve wide open and off its seat. This allows unlimited air removal. This feature is of an all-balanced pressure steam trap and it accounts for why these traps are suitable for air venting. 

Balanced Pressure thermostatic steam trap with a replaceable capsule.

When condensate flows through this trap, heat is transmitted to the liquid contained in the capsule. The liquid then vaporizes before the steam reaches the trap. The capsule vapor pressure makes it expand and which then makes the valve shut. Heat lost from the balanced thermostatic steam trap cools the water that is surrounding this capsule while vapor condenses causing the capsule to condense. This makes the valve open and releases condensate till when steam starts again and the cycle begins afresh. 

Working of a balanced pressure thermostatic steam trap

Currently, balanced pressure thermostatic steam trap manufacturers design these traps using stainless steel. Initially, thermostatic steam trap manufacturers used non-ferrous materials which were later found to be susceptible to water hammer damage. The use of stainless steel makes this trap more reliable and resistant to damage by corrosion, superheat, and water hammer. 

Bimetallic thermostatic steam trap 

This is a type of thermostatic steam trap made by the use of two strips of different metals that are welded together to form one element. This element can deflect if heated as shown in the figure below. 

Bimetallic thermostatic steam trap with its strip heated and at normal temperature

The simple element used in this type of trap considers two main factors that are:

• The operation of this bimetallic thermostatic steam trap occurs at a specific temperature that will not satisfy steam system requirements working at varying temperatures and pressure. 
• A single bimetal strip exerts small power. As such, a large mass needs to be used making it slow to react to changes in temperature in the steam. 

A steam trap performance can be determined by checking its response on a steam saturation curve. A suitable response is one closely following the curve and exactly below it. A basic bimetal element will react to changes in temperature in a linear manner. As shown in the figure below, some bimetallic thermostatic steam traps use two different bimetallic leaves in one stack that operates at different temperatures. 

A better bimetallic thermostatic steam trap employs the thermostatic element of a disk spring. The element consists of bimetal disks. If the disks act directly between the valve seat and the valve stem, they cause the condensate discharge temperature to change linearly as the pressure changes. When a spring washer is incorporated between a seat recess and disks it helps to absorb some of the expansion from the bimetal at low pressure. This helps to enhance a greater change in temperature as the pressure changes. The shape of the spring washer is preferred relative to coil spring because it can develop force exponentially which increases the rate compared to the linear rate. Such effect can take place for around 1.5 mega Pascal till the spring deflects to the bottom of the recess. This makes it possible for the condensate discharge temperature to follow the saturation curve of the steam more accurately. The rate of discharge can also be improved by the use of dynamic clack that tends to create a bang discharge. 

Bimetallic thermostatic steam trap with bimetallic disks

Applications of thermostatic steam trap 

• Bimetallic thermostatic steam traps are used in cooking kettles. 
• They are used in tracer lines. 
• These traps are used in sterilizers.
• Bimetallic thermostatic steam traps are used in convectors, radiators, and unit heaters. 
• They are used in heating coils. 
• They are used in evaporators.

Advantages and disadvantages of each type of thermostatic steam trap 

Advantages of liquid expansion thermostatic steam trap 

• This type of trap can be adjusted to eject at low temperature resulting in a superb cold drain facility. 
• When cold, the liquid expansion thermostatic steam trap gives excellent air discharge and high condensate volume on start-up load.
• Liquid expansion thermostatic steam trap can be used to start drain trap for low pressure and superheated steam mains in which there is a guarantee of a long cooling leg to flood cooler condensate. It will be able to overcome water hammer and vibration conditions. 

Disadvantages of liquid expansion thermostatic steam trap 

• In a liquid expansion thermostatic steam trap, the element has a flexible tube that can be destroyed in case of superheated or corrosive condensate. 
• This type of trap can eject discharge at below 100 ^oC temperature or below and thus it should not be used in areas that need immediate condensate removal from steam space. 
• If a liquid expansion thermostatic steam trap will experience freezing conditions, it should be well insulated as well as its piping system. 
• This type of trap will not offer a trapping solution alone, it needs another trap so they operate in parallel. However, it can serve in cases where the start-up flow rate is not the main factor like in draining heating coils in a small tank. 

Advantages of balanced pressure thermostatic steam trap 

• These steam traps are of large capacity, light, and small in size. 
• This type of thermostatic steam trap has its valve wholly opened on start-up. This makes it discharge air and non-condensable gases freely. This makes it possible for the removal of maximum condensate even at the greatest load. 
• Balanced pressure thermostatic steam trap rarely would freeze when operating in an exposed position except when condensate pipe level rises after the trap which causes water to run back flooding the trap when steam is turned off. 
• Balanced pressure thermostatic steam trap manufacturers design the trap with capabilities to automatically adjust itself to various steam pressures up to maximum working pressure. This trap is also able to withstand superheated temperatures up to 70 ^oC. 
• Maintenance of balanced pressure thermostatic steam trap is simple. This is because removal of the valve seat and capsule is easy and in case of replacement, it is done in a few minutes without the need to remove the trap from its line. 

Disadvantages of balanced pressure thermostatic steam trap 

• If the balanced pressure thermostatic steam trap is made up of non-ferrous material it is likely to be affected by water hammer. To ensure the trap is safe from water hammer buy one made of stainless steel material. 
• This type of trap will not open till the temperature of the condensate drops below steam temperature. This is disadvantageous in that if the selected steam trap is to be used where waterlogging of the steam area will not be tolerated such as heat exchangers, mains drainage, and critical tracing. 

Advantages of the bimetallic thermostatic steam trap

• Bimetallic thermostatic steam trap is designed with a compact body for strength and has a large capacity for condensate.
• A bimetallic thermostatic steam trap has a wide-open valve when it is cold. This gives the ability for air venting and capacity for maximum condensate discharge at start-up conditions.
• Condensate in a bimetallic thermostatic steam trap can drain freely from its outlet making this trap free from freezing up when operating in exposed position. Other bimetallic thermostatic steam trap manufacturers design the trap in a way that makes it not get damaged when freezing occurs.
• Bimetallic thermostatic steam trap manufacturers make the traps aiming to make them resistant to water hammers, high steam pressures, and condensate corrosion.
• The element in a bimetallic thermostatic steam trap can operate over an extensive range of steam pressure without needing a change in orifice valve size.
• If this trap has its valve on the downside of the seat, it tends to resist reverse flow. However, if a bimetallic thermostatic steam trap manufacturer expects some reverse flow, he/she will include a check valve at the downstream side.
• The condensate is discharged at various temperature levels below saturation temperature and the available waterlogging of the steam area can be endured, some of the saturated water enthalpies can be conveyed to the plant. This helps to draw out maximum steam energy from condensate before draining to waste. As such, bimetallic thermostatic steam traps are used in tracer lines. In tracer lines condensate is frequently dumped to waste.
• Maintenance of bimetallic thermostatic steam trap is easier because the internal components can be replaced without the need to remove body from its line.
• Flash steam created when condensate is ejected from high pressure to low pressure tends to cause increased back pressure in the condensate line. Because of the cooling leg, the condensate cools down producing smaller flash steam in the line of condensate. This helps to lower the backpressure.

Disadvantages of the bimetallic thermostatic steam trap

• Bimetallic thermostatic steam traps are not suited for use in process plants where it is necessary to have immediate removal to achieve maximum output.
• Some bimetallic thermostatic steam trap manufacturers design these traps with low internal velocities which makes them vulnerable to blockage from dirt in the pipeline. To help overcome such blockages, the trap needs to have dynamic cracks that lead to intermittent blast discharge which enhances self-cleaning capabilities.
• Bimetallic thermostatic steam traps respond slowly to changes in pressure or load as the element reacts slowly.

Troubleshooting thermostatic steam trap 

Thermostatic steam trap not discharging 

• This could be due to high pressure caused by the following: 

• Originally specified wrong pressure. Check the recommended pressure range. 
• Return line with high vacuum. This could increase differential pressure making it hard for the trap to work. Check the recommended value of the vacuum. 
• Increased pressure without a smaller orifice. Install smaller orifice. 
• Boiler gauge pressure is low. Increase the boiler gauge pressure. 

• Steam or condensate is not coming to the thermostatic steam trap

• Valve in line to trap is broken. Replace the valve as necessary. 
• Elbows or pipeline plugged. Unblock the elbow or pipeline as necessary. 
• Blocked by a strainer. Unblocked the strainer. 

• Defective or worn-out trap mechanism. 

• Check worn-out internal components and replace them as necessary. 

• The thermostatic steam trap body is full of dirt 

• Remove the dirt. Also, try to install a strainer ahead of the trap.  

Thermostatic steam trap losing steam 

• Valve not seating 

• This could be due to worn-out parts. Check worn-out parts and replace them as necessary. 
• Scales lodged in the orifice. Remove any material lodged in the orifice. 

Thermostatic steam trap experiencing continuous flow 

• The trap is very small. Use a larger trap. 
• Used high-pressure trap for a job needing low pressure. check the trap pressure requirement meets the job being done

Summary 

A Thermostatic steam trap is a trap that operates by use of the temperature difference principle between steam and cool condensate and air. Its steam pressure increases in a thermostatic element. This makes the trap close. To open this trap, condensate and non-condensable gases in the cooling leg back up which reduces temperature, and thus thermostatic element contracts making the valve open. There are three types of thermostatic steam traps which are balanced pressure thermostatic steam trap, liquid expansion thermostatic steam trap, and bimetallic thermostatic steam trap. Each of these three traps operates indifferently making each suitable for a specific type of application.

Thermostatic steam traps are manufactured using various strong and corrosion-resistant materials such as stainless steel, carbon steel, and bronze. These traps are used when handling light condensate loads. Thermostatic steam traps are used in various applications such as radiators, sterilizers, tracer lines, heating coils, evaporators, cooking kettles among others. These steam traps are advantageous in many dimensions such as having high efficiency, easy maintenance, and easier installation, ability to discharge air and other gases which reduces water hammer and can achieve high discharge rate among others.