Imagine this scenario: A power plant’s primary coolant pump loses power and is no longer providing coolant to the primary coolers. A steam sample in excess of 250°F continues to flow. In the chemistry lab, an operator works at a computer. A pipe hisses as the room begins to fill with dangerous steam. At the end of the sample tubes are costly analyzers about to be destroyed. The operator realizes what's happening and hurries to shut down the sample lines. The analyzers suffer minor damage but the operator is able to repair them. But, what about next time?
Sample temperature challenges
Loss of cooling water is only one of many situations that can put operators, instruments and the environment in danger. High sample temperatures can result from a number of other causes, including:
- Insufficient cooling water pressure or flow
- High cooling water temperature
- High sample flow rate
- A cooling water valve being accidentally closed
- Fouled or plugged sample cooler
Whatever the cause, sample lines must be shut down quickly to prevent harm to operators, instruments and the environment.
Obviously, plants understand that something must be done to ensure safety. Previously, the most common means of high temperature protection was a sensor – RTD or thermocouple – and an electrical solenoid valve. However, the solenoid valve has several drawbacks for this purpose:
- It requires electricity, which is not always available and can present an explosion hazard
- It is prone to sticking and burning out
- It is very large and expensive for high pressure lines
- The alarm signal is by electrical contact rather than by valve movement
- The magnetic field attracts corrosion-causing elements, mainly magnetite, which leads to sticking and plugging of the valve
An alternative solution uses pneumatic (air) actuators instead of electrical solenoid valves. This eliminates many of the undesirable features mentioned above – but air actuation is not always available in every plant situation, or reliable. Plus, a pneumatic actuator is quite large and outdoor lines can freeze if the air is not properly dried.
Yet another alternative is commercially available wax-operated valves. The wax seals the valve, and prevents steam from escaping, but when the valve cools down, it reopens. This can present greater hazards than the initial issue. A fourth and rather rare approach is the use of fusible wax operators. Upon use, or tripping, the wax is lost and must be replaced.
The dangerous situation described above need never happen again in any power plant. The best solution for stopping sample flow when there is a danger is the innovative Sentry® Thermal Shutoff Valve (TSV). The Sentry TSV is a self-contained device that can stop sample flow in less than five seconds. The valve’s sensor and actuator are directly exposed to the sample, so the valve can react immediately to any issues by ceasing sample flow. The valve needs no outside source of electricity, air, or hydraulics.
When sample temperature reaches the set point – the standard factory-set temperature is 120°F (49°C) – the valve closes and stops the sample flow. (Other temperature settings are available upon customer request.) A red indicator gives visual confirmation when a valve has tripped. If a remote alarm is preferred, an optional position-indicating dry contact can be attached to provide a signal in the field when the TSV has been tripped. After it has been tripped, the TSV must be manually reset, which provides operators positive closure and time to correct any issues in the line, as well as assurance that sample flow will not resume undetected. To provide maximum protection for operators and equipment, the optimum location for the TSV in the sample line is downstream of the pressure-reducing valve and upstream of the secondary cooler.
The Sentry Thermal Shutoff Valve (TSV) provides the protection and safety that every plant deserves. Learn more about the Sentry TSV here.
This article was originally published on our website.