Process Flow and Process Instrument Diagrams

To show a practical process example, let’s examine three diagrams for a compressor control system, beginning with a Process Flow Diagram, or PFD. In this fictitious process, water is being evaporated from a process solution under partial vacuum (provided by the compressor). The compressor then transports the vapors to a “knockout drum” where they condense into liquid form. As a typical PFD, this diagram shows the major interconnections of process vessels and equipment, but omits details such as instrument signal lines and auxiliary instruments:

One might guess the instrument interconnections based on the instruments’ labels. For instance, a good guess would be that the level transmitter (LT) on the bottom of the knockout drum might send the signal that eventually controls the level valve (LV) on the bottom of that same vessel. One might also guess that the temperature transmitter (TT) on the top of the evaporator might be part of the temperature control system that lets steam into the heating jacket of that vessel.

Based on this diagram alone, one would be hard-pressed to determine what control system, if any, controls the compressor itself. All the PFD shows relating directly to the compressor is a flow transmitter (FT) on the suction line. This level of uncertainty is perfectly acceptable for a PFD, because its purpose is merely to show the general flow of the process itself, and only a bare minimum of control instrumentation.

Process and Instrument Diagrams

The next level of detail is the Process and Instrument Diagram, or P&ID. Here, we see a “zooming in” of scope from the whole evaporator process to the compressor as a unit. The evaporator and knockout vessels almost fade into the background, with their associated instruments absent from view:

Now we see there is more instrumentation associated with the compressor than just a flow transmitter. There is also a differential pressure transmitter (PDT), a flow indicating controller (FIC), and a “recycle” control valve allowing some of the vapor coming out of the compressor’s discharge line to go back around into the compressor’s suction line. Additionally, we have a pair of temperature transmitters reporting suction and discharge line temperatures to an indicating recorder.

Some other noteworthy details emerge in the P&ID as well. We see that the flow transmitter, flow controller, pressure transmitter, and flow valve all bear a common number: 42. This common “loop number” indicates these four instruments are all part of the same control system. An instrument with any other loop number is part of a different control system, measuring and/or controlling some other function in the process. Examples of this include the two temperature transmitters and their respective recorders, bearing the loop numbers 41 and 43.

lease note the differences in the instrument “bubbles” as shown on this P&ID. Some of the bubbles are just open circles, where others have lines going through the middle. Each of these symbols has meaning according to the ISA (Instrumentation, Systems, and Automation society) standard:

The type of “bubble” used for each instrument tells us something about its location. This, obviously, is quite important when working in a facility with many thousands of instruments scattered over acres of facility area, structures, and buildings.

The rectangular box enclosing both temperature recorders shows they are part of the same physical instrument. In other words, this indicates there is really only one temperature recorder instrument, and that it plots both suction and discharge temperatures (most likely on the same trend graph). This suggests that each bubble may not necessarily represent a discrete, physical instrument, but rather an instrument function that may reside in a multi-function device.

Details we do not see on this P&ID include cable types, wire numbers, terminal blocks, junction boxes, instrument calibration ranges, failure modes, power sources, and the like. To examine this level of detail, we must turn to another document called a loop diagram (not in this post).

Reprinted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.

Worker Shortage in Engineering?

From the sales people on the front lines, to the engineering staff behind the scenes, to the managers in between, every staff member has a powerful influence on the success of a manufacturing company.

But are you facing difficulties in your manufacturing company in filling open positions? Are you finding a lack of skilled and qualified applicants? If the answer is, “yes,” you are not alone.

There are a multitude of factors contributing to the worker shortage in engineering starting with historically low unemployment, the aging population, and economic growth outpacing the rate at which jobs can be filled with trained workers. The talent pool shortage is expected to increase over the next 10 years.

Skilled Workers Aging and Retiring

The baby boomer retirement has been on the horizon for more than a decade, but the recession delayed some of its impact as older workers stayed in their jobs. High unemployment made it easy for businesses to find employees willing to work for less. A significant portion of the current manufacturing workforce is nearing retirement age and as these aged and skilled workers leave the workforce, they take critical industry knowledge with them.

STEM Skills Lacking

In recent years, the lack of STEM courses in high school has produced workers who are relatively unprepared for the demanding tech requirements of 21st century manufacturing jobs. Not only does STEM skills (math, science, and computer skills) prepare young workers for technical jobs, STEM also equips workers with problem solving skills, which is also a potential deficiency in individuals' capabilities for these jobs.

Many Students not Aware of the Industry

For the students who do possess strong STEM skills, they are often unaware of the career paths which would potentially match their skills. Industrial and Systems Engineering degrees are specifically designed to provide students with the skills to design and analyze automated manufacturing processes.

There is also a perception that engineering in manufacturing might not be hi-tech, it may be dangerous and dirty work, and may also be low salary positions. This means that younger workers may not consider the manufacturing industry as a career path.

The industrial sector is experiencing a manufacturing and engineer skills gap and it is up to the stakeholders to understand this critical situation and to look for innovative methods to fill jobs with experienced and knowledgeable engineers. Manufacturing work is increasingly technical, therefore these deficiencies in schooling need to be addressed. How can industry and associations work, in other ways, to bridge the gap?

One opportunity may be the developments in smart manufacturing and the industrial internet of things, connected enterprise strategies, Industry 4.0, robotics, data analytics and product design. These new technologies may logically appeal to the new generation of young people equipped with high tech skills. It points to industry associations and companies to effectively communicate these opportunities within the manufacturing and industrial segment.

Drexelbrook’s ThePoint™ Series Point Level Switch: Settings, Adjustments, and Changing Calibration Modes

The AMETEK Drexelbrook ThePoint™ Series uses No-Cal™ technology to detect the presence or absence of material without calibration or initiation via setpoint adjustments, push-buttons or magnets.

ThePoint™ level measurement switch features both Auto-Cal and manual calibration. The standard Auto-Calibration mode is applicable to most liquid and slurry point level measurements. If preferred, the manual calibration can be used and is recommended for some application. ThePoint electronic unit has auto and manual calibration modes built into the standard unit and can be accessed through the simple routine shown in the video below. The inclusion of these calibration modes allows the Drexelbrook RF Point Level Products application flexibility that is far greater than any other point level product on the market. ThePoint™ level switch can be used in Liquids, Solids, Slurries, and Interface applications.

To learn more specific instructions on the (8) calibration modes ThePoint™ has available, download the "Drexelbrook ThePoint Series Point Level Switch Installation and Operating Instructions" from this link.

Piping Specialties / PSI Controls: New England's Preferred Source for Industrial Valves, Valve Automation, and Process Instrumentation

Founded in 1975, with offices in Portland, Maine and Danvers, Massachusetts, PSI has earned their reputation as New England's premier supplier of industrial valves, valve automation, process instrumentation and specialty process equipment.

PSI specializes in engineered products for these industries:

• Power Generation
• Pulp & Paper
• LNG / LPG / Natural Gas / Gas Storage & Distribution
• Pharmaceutical / BioTech
• Food and Beverage
• HVAC
• Water & Wastewater

Piping Specialties, Inc / PSI Controls
https://psi-team.com
800-223-1468

Top 5 Considerations in Selecting Pressure Gauges

Pressure gauges are ubiquitous in industry and installed on machinery around the world. While there are millions of possible combinations of shapes, sizes, options and materials, careful attention needs to be given to the following five criteria for safe use and long product life.

1) Process Media
Direct exposure of the sensing element to the process media is a possibility, so attention must be given to any corrosive media, media with particulate, or media that can solidify and clog the pressure gauge element. For non-clogging media, a direct connection and Bourdon tube sensor can be applied. For process media that could potentially cause clogging, a diaphragm seal should be used.

2) Process Media Temperature

Gauge with
diaphragm seal.
(AMETEKUS Gauge)

Very hot media, such as steam or hot water, can elevate the gauge's internal temperature leading to failure or an unsafe condition. For high temperature application, the use of a gauge "siphon" or diaphragm seal be applied.

3) Ambient Operating Temperature and Environment
It is important to know the rated ambient environment for any instrument. Elevated ambient temperatures, moisture, vibration, and corrosive atmospheres can all affect accuracy, calibration, and safety. Choose the proper case and mechanism materials if oxidizing or reducing atmospheres exist, and consider the addition of ancillary devices, such as remote diaphragm seals, to relocate the gauge.

4) Possibility of Severe Pressure Fluctuations

In applications where dramatic line pulsations or strong over-pressure conditions are a possibility the use of pressure restrictors, snubbers, or liquid-filling will extend the service life of the pressure gauge.

5) Mounting

Snubbers & restrictors

Pressure gauges are standardly available with bottom (radial) and back connections. NPT (National Pipe Thread Taper) threaded connections are generally the standard. Many other process connections are available though, such as straight threads, metric threads, and specialized fittings. Make sure you know how the gauge is being connected. When mounting, pressure gauges should be almost always be mounted upright.

For more information about pressure gauges, contact PSI Controls by visiting https://psi-team.com or by calling 800-223-1468.

What is an External Spring, Lever & Weight Single Disc Check Valve

Single disk check valve animation.

According to Wikipedia, "Check valves are used in many fluid systems such as those in chemical and power plants, and in many other industrial processes."

Check valve symbol

As shown in the animation to the left, the single disc (swing) check valve use the directional flow to push open a swinging disk. As long as flow continues, the disk stays raised. But as flow stops, gravity allows the disk to re-seat itself and any reverse flow is prevented by the closed disk. As reverse flow pressure increases, the swing check valves seating increases as well.

Single disc check valves also use springs, levers and/or weights mounted on the valve to allow for better control of surge and to prevent the valve from slamming closed. These assemblies are used to vary the valve’s closing operation in order to reduce the severity of the closing water hammer.

External spring, external spring & lever, and external spring, level & weight designs (left to right).
Courtesy of Champion Valve.

For more information on any industrial check valve, contact Piping Specialties, Inc. by visiting https://psi-team.com or calling 800-223-1468.

An Introduction to Triple Offset Butterfly Valves

Triple Offset Butterfly Valve
(Pratt Industrial)

Triple offset butterfly valves are designed to fill the demand for an alternate solution to gate valves and ball valves. They are preferred when weight, space, performance, and the ability to modulate to the process flow are an issue. Typically available in sizes 3" through 48", and in 150, 300 and 600 pressure classes, they're rated for operation from -50 deg. F. through 750 deg. F.

Designed with metal-to metal seats, triple offset butterfly valves provide distinct advantages over traditional gate valves, namely lower weight, zero-leakage, ease in automation, and capable of being used for modulating service.

Industries Using 

• Chemical
• Refinery
• Power
• Steam Generation
• Petrochemical
• Water/Waste Water Treatment

Click on image for larger view.

Triple Eccentric Disc-Shaft Design (diagram above)

• Offset 1: It is accomplished by moving the centerline of the shaft away from the seating plane.
• Offset 2: It is accomplished by moving the centerline of the shaft offset from the centerline bore of the valve.
• These two design features cause the disc to open and close relative to the body seat in a “camming” action and effectuate the position seated valve design which is typical of the High Performance Butterfly Valve, however there is still contact between the disc and the seat in the first several degrees of opening and closing which can cause premature wear of the seat in the general areas.
• In order to achieve an API 598 Shut Off classification a 3rd offset needed to be introduced to make the valve a “torque seated” design with graphite and metal seating surfaces.
• Offset 3: It is accomplished by adjusting the cone angle created by the 1st and 2nd offset angles at some point downstream of the valve in the center of the piping to the adjacent piping wall as depicted in the illustration below “Sticking tendency”. By incorporating the 3 offsets into one design typical of gate valves is eliminated with seat contact throughout the entire stroke reducing run torques and improving actuator modulating performance at the same time.

For more information, contact Piping Specialties by visiting https://psi-team.com or by calling 800-223-1468.