Pneumatic Instruments

Pneumatic transmitters
(courtesy of Foxboro)

Air pressure may be used as an alternative signaling medium to electricity. Imagine a pressure transmitter designed to output a variable air pressure according to its calibration rather than a variable electric current. Such a transmitter would have to be supplied with a source of constant-pressure compressed air instead of an electric voltage, and the resulting output signal would be conveyed to the indicator via tubing instead of wires:

The indicator in this case would be a special pressure gauge, calibrated to read in units of process pressure although actuated by the pressure of clean compressed air from the transmitter instead of directly by process fluid. The most common range of air pressure for industrial pneumatic instruments is 3 to 15 PSI. An output pressure of 3 PSI represents the low end of the process measurement scale and an output pressure of 15 PSI represents the high end of the measurement scale. Applied to the previous example of a transmitter calibrated to a range of 0 to 250 PSI, a lack of process pressure would result in the transmitter outputting a 3 PSI air signal and full process pressure would result in an air signal of 15 PSI. The face of this special “receiver” gauge would be labeled from 0 to 250 PSI, while the actual mechanism would operate on the 3 to 15 PSI range output by the transmitter. As with the 4-20 mA loop, the end-user need not know how the information gets transmitted from the process to the indicator. The 3-15 PSI signal medium is once again transparent to the operator.

Typically, a 3 PSI pressure value represents 0% of scale, a 15 PSI pressure value represents 100% of scale, and any pressure value in between 3 and 15 PSI represents a commensurate percentage in between 0% and 100%. The following table shows the corresponding current and percentage values for each 25% increment between 0% and 100%. Every instrument technician tasked with maintaining 3-15 PSI pneumatic instruments commits these values to memory, because they are referenced so often:

Using the Foxboro model 13A pneumatic differential pressure transmitter as an example, the video below highlights the major design elements of pneumatic transmitters, including an overview of "maximum working pressure" versus "maximum measurement range" pressure.

The Foxboro model 13A pneumatic d/p cell transmitters measure differential pressure and transmit a proportional pneumatic output signal.

The information above is attributed to Tony Kuphaldt and is licensed under the Creative Commons Attribution 3.0.

Mead O'Brien: Steam and Hot Water System Experts

Let Mead O’Brien help you create a sustainable Steam Trap Management Process!

• Trained Survey Technicians 
• Traps located and identified, tagged with SS tag #, and data logged with up to 27 fields of useful data per trap 
• Executive summary, Failed trap report with steam & dollar losses, detailed Log sheets, and Recommendations are all provided in a professional report. 
• Monitoring options presented for critical service applications 
• Steam flow measurement design 
• Heat recovery potential 
• Training options in a live steam lab 

Realize the Savings Now!

• Reduce steam & condensate losses 
• Reduce loss of boiler chemicals 
• Improve heat transfer performance 
• Prevent coil and heat exchanger damage 
• Minimize water hammer hazards 

Mead O’Brien and Armstrong, more than 85 years of Steam & Hot Water System Optimization

• Steam Distribution
• Process Heat Transfer and Control
• Condensate Return
• Heat Recovery Opportunities
• Process, Ambient & Combustion Air
• Steam Trap Surveys & Database Creation 
• Humidification Assessment
• Application issues
• - Coil Freezing Issues
• - Poor Heat Transfer & Steam Control - Water Hammer Issues
• - High Backpressure
• Steam & Condensate Measurements, Control & Monitoring

Learning Systems:

• Armstrong University
• Over 125 web-based courses 
• Mead O’Brien Live Steam Lab
• Content Tailored for Plant Need

Steam Trap Testing Guide for Energy Conservation

Annual steam trap testing schedule

Below is a steam trap testing guide (courtesy of Armstrong International) to maximize efficiency and conserve energy. This guide discusses:

• Steam Trap Testing Procedure 
• Tips On Listening 
• Inverted Bucket 
• Float & Thermostatic Trap 
• Disc Trap 
• Thermostatic Trap 
• Sub-Cooling Trap 
• Traps on Superheated Steam

CAUTION: Valves in steam lines should be opened or closed by authorized personnel only, following the correct procedure for specific system conditions. Always isolate steam trap from pressurized supply and return lines before opening for inspection or repair. Isolate strainer from pressurized system before opening to clean. Failure to follow correct procedures can result in system damage and possible bodily injury.

Steam Trap Testing Guide for Energy Conservation from Mead O'Brien, Inc.

Steam System Condensate - Save Big by Managing its Proper Return

Recover/return steam condensate

An often overlooked place to find savings in the operation of a manufacturing plant is the steam condensate return system. Returning condensate to the boiler feedwater system saves energy by returning pre-heated water, thus requiring less energy to maintain the feed water temperature. Furthermore, condensate is pre-treated, eliminating the need for additional expensive treatment.

Steam use in modern plants is everywhere. The condensate derived from its use is an asset that needs to be recycled. Older steam systems may have poor condensate return piping, or none at all. For these situations, creating or extending return lines should be considered. Steam traps in older systems are also suspect. Older traps are not as efficient or reliable as newer designs.

According to Armstrong International’s Senior Utility Systems Engineer Novena Iordanova, steam traps are very important in the condensate return/feedwater cycle. She believes the first purpose of a steam system is to deliver steam to a users defined area of need, and second, to return the resulting condensate back to the boiler. Unfortunately, in many cases the second goal is overlooked.

In older plants, a complete steam system evaluation, and many times an overhaul, is required. Professionals should be called in as plant maintenance staff typically doesn’t have the expertise required. The reasons for steam system degradation are common in the life cycle of plant operation. Over time, plant upgrades, new lines, expansion, and new equipment can have a significant detrimental effect on condensate return systems. The focus usually goes to the main headers and distribution, but condensate return doesn’t receive the same attention. The result can be a steam system that is no longer efficient - meaning high back pressures, water hammer, broken or freezing pipes, and leaks. At that point, the plant needs professional help for a complete system review.

Steam trap
(courtesy of Armstrong)

The most important player in condensate recovery is the steam trap. Steam traps play the critical role of separating steam and condensate at the moment its formed. Traps discharge the condensate downstream to the boiler for reuse. Proper sizing, installation, and maintenance from the start will save huge amounts of money in terms of energy savings and maintenance time.

Frequent and regular inspections of steam traps are essential, but in reality they are many times postponed or rescheduled. Steam trap inspection is tough work. Traps are usually located in hard to see, hot, and tight areas. Over time, the difficulty (combined with the perceived low priority) degenerates to spotty inspection routines, higher trap fail rates and higher steam costs.

The good news is that steam trap monitoring systems now exist that can monitor the performance of steam traps and alert maintenance when things start to deteriorate. Steam trap monitoring systems report conditions that point to filature, and also alarm when things break down. Accordingly, maintenance is in a position to take preventive action, or make swift repairs. Any plant with a considerable number of stream traps should strongly consider deploying a steam trap monitoring system.

Steam trap monitoring systems monitor the thermal and acoustic characteristic of the trap and report any significant changes. Today many monitoring systems are wireless, and many operate on common plant communication systems such as WirelessHART, a communication protocol gaining worldwide acceptance.

Its important to mention here though, the on the most common mistake plant personnel make when it comes to their system is also one of  the most obvious - insulation. The use of high quality, well maintained insulation, installed to allow access to steam components is critical. The energy savings alone from well insulated pipes and traps is argument enough for making the initial time and dollar investment for proper insulating.

If you’re a plant manager and are looking for significant measurable and meaningful ways to lower energy costs, you must consider a well planned and well executed steam trap management program.