In order to correctly diagnose problems, we must have a good understanding of the heat exchanger and components that make up the overall system.
The first part of this series described system function and stall. This installment will look at system components, back pressure, and the effects of stall.
The entire series describes shell-and-tube heat exchangers, although most elements of our system will apply to all types of steam heat exchangers.
Specific system componentsLet’s look at some of the system components before and after our exchanger to help us better understand stall conditions.
The majority of steam heat exchangers use a temperature control valve to control the process variable or “fluid discharge temperature.” This valve can be an on-off or a modulating type.
On-off valves can be used for instantaneous hot water, but are not recommended where close temperature control is required.
The first choice for close temperature control usually is a modulating valve. Should the system experience poor temperature control, this valve becomes the first suspect.
While the valve may be the cause, other problem areas can have the same effect on temperature control.
Many types of these modulating valves on the market vary in accuracy, capability, and principle of operation. The most common valve models are actuated by self-contained, capillary-type thermal systems, electrically actuated, or pneumatically actuated. All differ in sensitivity and accuracy.
It is essential that these valves are properly sized for the inlet and outlet pressures, or they will cause the system to “temperature hunt,” or prevent it from reaching setpoint.
Properly sizing and selecting a temperature control valve is a science in itself due to factors such as turndown ratio, proportional band, accuracy, and others. (Control valve selection warrants a discussion of its own, but for our purposes, we will consider the control valve as properly selected.)
Another critical part of the steam exchanger system is the steam trap. Members of the industry have divergent opinions regarding the type of steam trap best suited for heat exchanger applications.
Among the types commonly recommended are bucket, thermostatic, and float and thermostatic. Several manufacturers offer a variety of traps and each has its advantages.
After consideration of flow capability and working pressure within the system, the most popular types of traps used on steam heat exchangers are the float and thermostatic. Advantages of these traps include:
- High air venting capabilities — This is very important, especially on start-up as the air inside the exchanger must be exhausted before steam can enter.
- Modulation style of discharge — This allows for condensate to be discharged continuously as it is formed and is less likely to cause sudden pressure drops inside the exchanger.
- Condensate barrier surrounding discharge orifice — This helps ensure that live steam cannot pass through the steam trap as the orifice and plug operate under the condensate level inside the steam trap.
- Works well under low loads as well as full loads — A high turndown ratio makes it especially attractive with systems that use a modulating steam control valve.
- Available in a variety of sizes that can handle many variations in pressure and capabilities.
Regardless of the type of steam trap chosen, it must be sized to handle the full-load condensing rate without backing up condensate. A steam trap acts like an automatic valve that holds back steam while discharging condensate. It does not pump condensate.
Ideally, the steam trap will not see back pressure if the discharge from the trap flows by gravity through a generously sized pipe and down into a vented receiver.
Back pressureHowever, the previously described scenario isn’t the case in many installations. Often, the steam trap must overcome back pressures that the system has introduced.
These back pressures may be created by any of the following:
- Condensate lifts, where the trap must discharge into an overhead return line (2-ft lift = 1-psi pressure);
- The common condensate return line may have other traps that have failed while open, thereby pressuring the return line and causing back pressure;
- Return line discharging into a pressurized deaerator; or
- An undersized common return line unable to handle the flash and condensate loads is forcing back pressure on all traps discharging into it.
Back pressure needs to be taken into account during the design of a system. Some designers may make a fundamental error when considering the steam pressure (e.g., 15-psi steam will be able to overcome a back pressure of a 10-ft lift [5 psi] after the trap).
Others may consider increasing the inlet steam pressure to overcome the possible back pressure. However, this can lead to other problems, since higher pressure steam consumes more pounds per hour due to the lower latent heat content.
Employing a higher steam pressure creates a lot more flash steam downstream of the trap as high-pressure condensate is introduced to a lower pressure return line. Unless this flash steam is recovered, money is wasted as low-pressure flash steam eventually is vented out to the atmosphere.
Effects of stallNeglecting the effects of system stall can be still more critical. Regardless of the pressure of steam supplied to the modulating control valve, a system will stall at some point with any given steam pressure. That point will be decided by the level of back pressure in the system.
As the load decreases, so does the demand for steam. Soon, the modulating valve begins to pinch down, lowering the steam pressure available to push condensate from the trap.
Under low-load conditions, or when the control valve is closed completely, a vacuum will form inside the heat exchanger as steam collapses to a much smaller volume of condensate.
If the condensate is allowed to discharge by gravity into a vented receiver (0 back pressure), a vacuum breaker will open, exposing the exchanger to atmosphere pressure. A vertical distance of 15 in. from the bottom of the exchanger to the top of trap will provide about 0.5-psi hydrostatic head for condensate to drain from the steam.
Ideally, if the steam exchanger is installed with no back pressure on the steam trap, the stall point is not an issue because condensate can drain freely. It is important to note that in many installations, the steam trap must discharge against a back pressure.
Now, when the steam control valve reduces its flow, the system will stall when the steam inlet pressure equals the back pressure against the trap. This will happen at a pressure above the vacuum.
Thus, the vacuum breaker won’t help relieve stall under this condition and condensate will back up into the body of the heat exchanger. Higher back pressures will create a system stall much sooner (for example, at higher loads), and a study of the condensate return system is needed.
Even if a higher steam inlet pressure is used, the modulating valve will pinch down and eliminate the differential needed across the trap to discharge the condensate since the load has decreased the need for steam flow — as it is supposed to do.
This stall point will take longer to reach than at lower steam inlet pressures, but will still occur. As a result, increasing the steam pressure is not the solution.
Other factors that contribute to system stall include overgenerous fouling factors and equipment overseeing due to “fudge” or safety factors.
These factors are amplified when the exchanger is expected to run at high turndown, which means operating the exchanger below design conditions.
The best solution for system stall is draining condensate from the trap by gravity to a vented receiver. In this case, stall will occur at 0-psi steam pressure. When vacuum forms, it will be broken by the vacuum breaker and the 0.5-psi static head created before the trap (for example, 15-in. vertical distance between trap and bottom of heat exchanger) will force the condensate through the trap.
A safety factor of 1.5 times the calculated condensate load should be added to ensure condensate removal.
Sidebar: In a nutshell
- Many types of steam heat exchangers use similar controls and principles of operation.
- Common problems include poor temperature control, water hammer, premature corrosion, and freezing.
- Using higher steam inlet pressure decreases the stall point without rectifying the problem. Higher steam pressures also pose steam losses through lower latent heat content and flash loss.
- To avoid system flooding due to stall, the steam trap must discharge by gravity into a vented receiver, thereby never exposing the system to back pressure.