Industrial Utility Efficiency    

Reliability: Should Compressed Air Monitoring be Combined with Control?

Many OEMs of air compressors, dryers, sensors and master controls are integrating monitoring features and capabilities into their components. It would seem a no-brainer to keep it simple and use those sensors and systems for both control and monitoring. What could be simpler? 

While there are many individual stories that can be told by each of these equipment and component vendors that show it is easy and useful to do that type of data and control integration, there are problems that are not being addressed. The problems have to do with system reliability and integration. Things need to be simple and robust enough to work reliably for their primary purpose, which might either be control or monitoring. And integration issues need to be carefully addressed so the data side can’t trump the control side, and the control side can’t trump the data side. 

Before you purchase an integrated control and monitoring system for compressed air, you need to understand some basic issues specific to your plant (or your customer’s plant). Specifically, people and problems. What people are the internal customers of the monitoring and controls? What are their concerns, tools and limitations?  And what problems can happen in the real plant world, and what consequences to reliability are there? 

General industry’s most scarce asset is people. Who are the internal customers of a compressed air control system, who I will call “Controls Customers,” and who are customers of a compressed air monitoring system, or “Monitoring Customers?” From my experience, they are very different people in typical industrial organizations.

 

Internal Customers of Compressed Air Controls

In general industry, a control system for compressed air is not generally seen as part of production automation or process control. It is usually seen as part of an internal utility, and managed by the utilities team. In some plants, this is team is called “Facility Management,” which includes Facility Engineering (design and support of changes to buildings and processes) and Maintenance (keeping utilities and processes repaired).  In others, Maintenance is the Controls Customer. In large process plants, the customer might be called “Utilities.” 

What are the major concerns of the Controls Customer? Reliability is their primary concern. If plant air needs require an additional air compressor to come online, they need to know that will happen every time. If needs reduce and it’s no longer needed, they have little concern to turn it off.  That is addressed by the next customer we will discuss. The Controls Customer is spooked by complex controls that they can’t predict. They prefer algorithms that operate in predictable patterns. If “X” occurs, “Y” happens. If “A” fails, “B” backs it up. They also prefer simplicity because they have learned the hard way that the more points of failure, the more failure will happen.

What kinds of monitoring does a Controls Customer want to look at? Real-time monitoring of machine health. They want to know the current value of any process variable that could shut the machine down, and any that affects the output that they need.

Why am I bringing up these Controls Customers first? Because they really don’t value long-term monitoring very much, if at all. They have no time for energy management or performance issues, though they typically care about it. 

 

Internal Customers of Compressed Air Monitoring

With the advent of Energy Management Control Systems (EMCS) in HVAC and building management systems (BMS), a new category of internal customers started requiring performance information from the plant’s systems. These systems are offered to pretty much every new commercial building as a standard practice. In industrial firms that have EMCS’s in their buildings, often EMCS systems existed before the compressed air control system was installed, if there even is one. 

Who looks at the trend data? Who is the customer of the EMCS? Typically, a person responsible for building and/or utilities performance. The energy engineer, utilities manager, or intern who supports them would typically be the customer of the EMCS.  Since compressed air monitoring systems are so infrequently installed in the United States, that person is likely going to be the customer of a compressed air monitoring system. Why not the Utilities or Facility department that owns the equipment? Although it seems logical, those staff members are not accustomed to using trend data for performance optimization. And they aren’t generally even at their desks long enough to do so if they did have the inclination and skills. The Facility department puts in the EMCS but rarely looks at it.

What are the concerns of the Monitoring Customers?  In a word, performance. They want to see that the system is working optimally and providing the required output at the minimum energy input. They look at macro-level data, and compare current performance to previous months and years, and in comparison, to a baseline and a benchmark.  Compressed air is just one important part of a big picture they look at.  

 

Understanding the Potential Problems

There are two types of potential problems with combining control and monitoring systems, reliability and integration. This article will discuss reliability. The next article in this two-part series will discuss data integration issues.

There are three sources of reliability problems in any compressed air system: Murphy, Bubba and “the Powers that Be.” Murphy says the smallest item that was the last thing the designer thought of will be your first problem that will shut your whole system down.  Bubba will never read instructions and will do things to your system that the designer never dreamed. And the Powers that Be are the Powers that Be. Things happen out of your control you can just never anticipate.

How do you minimize the impact of these three reliability problems? Have a simpler system with fewer points of failure that can provide close to ideal performance, reliably.  I'll provide examples, starting with a real example of a reliable control system. 

A sawmill used a PLC-based sequencer with three fixed-speed air compressors. Only one process variable was needed to control the air compressors: pressure, which is provided by an almost unbreakable simple component: a pressure transmitter. It is intolerant to temperature, vibration, and contamination. The PLC was an industry-standard unit, built for an industrial environment, unlike a PC or most EMCS components. It can endure higher temperatures, voltage spikes and dips, and more. As shown in Figure 1, the control system displayed which stage the system was in, and what would be next if load went up or down. The control system did not do any trend logging. Why? The reliable hardware software available in the acceptable price range for the customer did not have that functionality. 

Sawmill Control System

 Figure 1: A typical PLC-based control system screen.

 

Energy Company Uses Too Many Process Variables

Another example of an unreliable control system is one that tried to use too many process variables, in ways that can be unpredictable. I audited a system at an energy company with a custom master controller that used a combination of flow, motor current, and two pressures to control four air compressors and a booster. The flow meter was an inexpensive thermal mass meter that could read high if the dryer malfunctioned (wet air pegs these meters). Current was used to estimate percent load on an air compressor. And a flawed algorithm with 27 different inputs (many combinations thereof which were illogical) was used to set up air compressor staging. 

EMCS screen

Shown is a typical EMCS screen for an HVAC system. Click here to enlarge.

Bubba and Murphy could both have a heyday with this system. This "flow-based" system attempted to control different combinations of air compressors in different flow ranges.  It had a flawed algorithm that had no allowance for flow meter drift or air compressor local controls being misadjusted. Recall the primary problem in the Boeing 737MAX fiasco - reliance on one flaky sensor that can jam.

Flow should be used for monitoring, not control. A "flow-based" control system I recommended in a previous article for some large systems would use calculated flow, based on a reliable air compressor percent flow metric like speed or current. I saw this energy company's attempt at flow-based control about seven years after I wrote that article. Maybe they designed it after they read it! To read the previous article, visit https://www.airbestpractices.com/system-assessments/compressor-controls/compressor-sequencer-problems-and-solutions.

Unfortunately, flow meters can fail in many more ways that pressure transducers, and can read inaccurately even if not failed, based on installation issues, vibration, temperature, and fluid quality. A control system that uses them as a process variable is unreliable.  And one that has too many operator inputs is too prone to being "hacked," resulting in erratic, unreliable operation.

Another reliability issue I warn about is the use of the EMCS network backbone for industrial control. EMCS systems are typically run over the building general Ethernet, often the same network used for office computers. Although reliability is important for that network, the consequences of it going down are usually not catastrophic. The office might get warm, the energy engineer might not see performance for a while, or an engineer might have to change computers temporarily. I have seen the EMCS network at an aerospace company have constant problems with hubs that converted Modbus signals to Ethernet. They failed often, and took down banks of sensors when they failed. If that system had been used for air compressor control, the plant would have gone down.

 

System Monitoring Recommendations

When considering how to add monitoring to your compressed air system, consider the people and problems associated with the complexity of mixing controls and monitoring. Align the technology with the people and tools available, and keep the points of failure to a minimum. Here are some recommendations, not hard rules:

  1. For small and mid-sized systems, consider simply adding a flow meter, power (or current) meters, pressure transducer and dewpoint meter to a simple, permanently installed, separate data-logging and trending system rather than attempting to integrate monitoring and control. You can get a lower-cost monitoring system installed quickly this way. You can also get the monitoring system installed earlier, and use it for commissioning. There are many options for portable data-loggers, but you need a permanent logging system, which is very different. It should have an Ethernet connection and the ability to be accessed remotely and to integrate in external databases. Several products are available. Two I am aware of are from VP Instruments and Airmatics. 
  2. If you have an engineering function within Facilities or Maintenance that regularly looks at data trends and uses tools like Excel to analyze and display it, you have an internal Monitoring Customer for compressed air monitoring. If you don’t, I would not recommend a monitoring system at all, unless an outside party can help as explained below.
  3. If you have an EMCS already, you have both a system and a Monitoring Customer.  Consider having separate monitoring and control systems for compressed air. A reliable sequencer, offered by your OEM or an expert third-party, can do a good job of control with the least number of failure points. Then, add compressed air sensors to your EMCS and develop monitoring screens on the EMCS, and data trending as well. Use a compressed air expert’s design for that monitoring system. 
  4. If you don’t have either a local engineer who can look at and analyze data or an EMCS, but management needs performance information, consider a monitoring system that is “cloud-based.” Most new air compressor and dryer suppliers are offering low-cost remote monitoring. Your supplier should be capable of using those systems and providing service via that data.
  5. For large systems, consider hiring an integrator who can custom design a control and monitoring system that does the best of both, using plant standards for both, serving the separate Control and Monitoring customers. A compressed air expert should guide the integrator.

 

For more information, contact Tim Dugan, President of Compression Engineering Corporation, tel: 503-520-0700; email: Tim.Dugan@comp-eng.com, or visit http://www.compression-engineering.com/.

To read the second part of this article, Integration: Should Compressed Air Monitoring be Combined with Control, click here.

To read more Air Compressor Controls System Assessments articles, please visit www.airbestpractices.com/system-assessments.