Electromechanical Pressure, Differential Pressure and Temperature Switches - The Basics

Most industrial applications require the monitoring of pressure and temperature of a process. Pressure and temperature measurement can be accomplished either by transmitters, gauges or by switches. This post will provide a quick introduction of industrial electromechanical pressure switches and temperature switches.

An industrial pressure and temperature switch is made up of the three main components: 1) the sensor, 2) the housing and 3) the switching element.

The correct combination of each component assures proper application of the device for its intended use.

Natural Draft and Mechanical Draft Cooling Tower Fundamentals

Cooling Tower
Delta Cooling Tower
In many industrial facilities, various pieces of equipment, as well as many fluids used in process systems need to be cooled.  This cooling is mostly done with water. However, as cooling water is used, it absorbs heat, and loses its cooling effectiveness. The water needs to be kept cool.

Disposing of hot water in to ponds or basins can be detrimental to the environment. It’s also costly to replace the discharged water. The more efficient means is to cool the hot water and reuse it.

The equipment most commonly used to do this is the cooling tower. Cooling towers are part of a cooling water system in a commercial or industrial facility.

The main components of a typical cooling tower are a circulating pump, a shell and tube heat exchanger, and fluid lines.

Pressure Reducing Valves for Steam Opimization

pressure reducing valve
Pressure Reducing
Valve (PRV)
A well designed steam system should produce clean, dry steam ready for distribution at high pressure through the steam distribution network. This maximizes the potential to generate and supply quality saturated steam at the lowest total cost.

Most applications require a pressure reduction at the point of use.

Significant benefits include:

1) A reduction in the cost of capital equipment; 2) Plant costs decreases by reducing flash steam; 3) Since saturated steam pressure is directly related to temperature, controlling pressure will automatically control temperature thus avoiding the need for supplemental temperature controls; and 4) The ability to supply optimized steam pressure for any individual application.

Weld In-Place Ball Valves - A Unique Design Approach

Weld In-Place Ball Valve
Weld In-Place Ball Valve
Historically, weld-end ball valves presented challenges to users in process control applications because of the high temperatures during the weld, which would damage the temperature sensitive parts of the valve (seals and seats).

One work-around is to extend the piping to the valves ends, but this adds cost and time. Another solution is to dis-assemble the valves and remove the seals and seats prior to welding. Then, after welding, re-assembling the valve when the valve after everything has cooled down. This is much more complicated, takes more time and can be very problematic if the valve is part of an automated package.

A unique approach for socket weld and three piece valves, pioneered by valve manufacturer Flow-Tite, that uses integrated extended end-caps with heat sink rings. The design provides much more surface area, thus allowing the heat to dissipate during welding. Any heat conducted to the seat area does not have a high enough temperature to damage the valve seating or sealing material.

With this novel and common sense approach approach, soft-seated, three-piece ball valves that were once a problem to weld, can now be welded in-place without disassembly, extended time and related costs.

Basics of Safety Relief Valves

Kunkle Relief Valve
Typical Safety Relief Valve
Gases and steam are compressible. It is normal that when gas reaches the disc in a valve, it compresses and builds up before passing through the valve. This compression may cause a rapid build up of system pressure and be potentially harmful.

A conventional liquid type relief type relief valve doesn't open fast enough to relieve gas or steam pressure. The slower action may actually contribute to pressure build-up. A compressible gas system requires a valve that will pop wide open under excessive pressure. That's the design principle behind a pressure safety valve also known as a PSV.

Safety relieve valves and relief valves are similar and share common design and components. The direct acting safety valve is made up of a inlet, outlet, housing, disk, seat, spindle, a cap, and in some instances, a manual operating lever. The safety valve assembly is protected by the housing which has a threaded or flanged pipe connection to the system. The cap protects the top of the valve and reduces the chance of inadvertently changing the valve setting. The disk stays in place until the system pressure increases to the point when the disk “pops” off the seat. The spindle aligns the disk. An adjusting screw is used to set the valves' set point or popping pressure. Spring tension can change over time an require the recalibration of the adjusting screw.

Explaining Heat Exchanger Stall

Spirax APT
Spirax Sarco APT
The most common process heating, heat exchanger hookup uses a temperature control valve on the steam line to the heat exchanger, and a steam trap on the condensate line from the heat exchanger.

The shell side is this steam space. A control sensor signal at the tube side outlet is used to throttle the steam control valve to maintain set point temperature. Higher pressure in the steam space than in the condensate recovery line produces effective condensate removal and lift to the return system.

Under a steady high load, differential pressure removes the condensate from the heat exchanger. Under reduced heating load, the control valve throttles down, reducing the steam pressure inside the heat exchanger. This also reduces the differential pressure across the steam trap making the trap unable to remove the condensate. This happens in all heat exchangers, whether properly sized or oversized.

This causes condensate to flood the steam space, known as heat exchanger stall. In other words, the pressure in the heat exchanger is equal to, or less than, the total back-pressure imposed on the steam trap, sometimes even attaining vacuum.

Some operators address vacuum in the steam space by installing a vacuum breaker on the shell. This practice introduces atmospheric gases that dissolve readily into the cooler condensate. These dissolved gases form corrosives that attack wetted surfaces, while doing nothing to eliminate the stall condition.

The simplest way to cure stall is to install a steam-powered automatic pump trap, such as a Spirax Sarco APT series. Pump trap operation is based on condensate level alone, with live steam pressure removing condensate under all load conditions, even vacuum.

By not using a vacuum breaker, you can reduce condensate acidity and large temperature swings in the heat exchange equipment. Heat transfer and control improve. Corrosion, water hammer, tube failure, excessive treatment chemical dosing, and high maintenance costs become distant memories.

A survey of your heat exchanger and condensate return system operating and maintenance data can uncover the below-par performance that indicates stall. If present an automatic pump trap is an easy solution that quickly returns dividends in process quality, energy savings, and reduced maintenance costs.

For more information on how to prevent heat exchanger stall, contact Mountain States Engineering and Controls at 303-232-4100 or visit www.mnteng.com.

A Specialty Electric Actuator for Linear Valves with Precise Control and Failsafe

Warren Controls’ compact AMURACT electrically operates 1/2” through 4”control valves quickly, reliably, accurately and with shutoff capability rivaling pneumatics. Works with multiple supply voltages and signals. Electronic fail-safe needs no operating force to maintain lockup against elevated water or steam pressures. Stainless steel construction is maintenance-free. Purchase price is competitive and operating costs are extremely low. Here's a demo: