Industrial Utility Efficiency    

OEM MACHINE DESIGN. Pneumatics: Sizing Demand Users

Most systems are sized on the supply side at many times more volume and significantly higher pressure than is actually necessary to support the real demand plus a fudge factor generally created out of fear. I am sure that had the OEM defined what is not only minimally necessary in terms of mass flow at density (pressure and temperature), but also with the intent of the highest possible efficiency, we would approach things very differently. Unfortunately, what drives the design and arrangement generally involves three goals for the OEM:

a. Size of components

b. Cost of components

c. Workability of the application

To provide an example of what happens, most systems are diagnosed as having at least some undersized piping. Whenever I begin an audit, the client usually tells me that the system’s piping is old and never been reevaluated. When I ask how he or she knows that the system is under sized, I generally get a somewhat similar answer. I am told that there is an air user at the far end of the system that cannot hold pressure. I am told that when the supply system is at 100 psig, the user can’t hold 85 psig. When I ask the client how he knows that he needs 85 psig and where, I am generally told that 85 psig is the right number because the application doesn't work below 85 and he needs 85 at the gauge at the application. Let’s look at the application arrangement:

FOSSIlustration1

Please note that the gauge pressure is being read at the downstream side of the regulator and the upstream side of the hose, hose fittings, and generally, the lubricator. When the application is static or off, the pressure read is the same as the supply pressure assuming leaks are minimal. When the application is activated, the gauge pressure reflects the differential across the filter including dirt loading on the filter element plus the delta pressure on the regulator. In most cases the total differential equals what is believed to be the differential across the piping If we changed the piping, there would be no material impact on the supply pressure to the user....perhaps 1-2 psig. The bulk of the delta is across the filter/regulator. The application probably has 10-15 psid across the hose, hose fittings, and the lubricator. This means that the article pressure at mass flow is <70 psig. You can confirm this with either a needle guage or a “T” and female Pete's Plug with a gauge in it located at the air user inlet. You can also use two pressure transducers, one at the dischasrge from the compressor room and one at the gauge port on the upstream side of the regulator plus a local gauge on the downstream side of the regulator. You can also install another analog gauge at the artifle pressure location in a female Pete's Plug.. Please keep in mind that most analog gauges are cheap and can be off by as nuch as 6-7 psig. Make sure they are calabrateable and are feild tested for accuracy.

mostOEMObviously, resizing the components to a total of 10 psid makes more sense than jacking up the system by 10-20 psig. Unfortunately quick and inappropriate decisions are made by many maintenance personnel to simply elevate the systems pressure as much as possible over time. One of the problems with jacking up the supply pressure is that as the pressure rises, the demand flow on unregulated demand increases proportionately. The differential pressure increases as a square function of flow change. The result will provide a diminishing return as you jack up the pressure on all unregulated flows including the point of use applications with regulators that are wide open and waste such as leaks. . In most plants we find that 80% of all point of use applications have regulators that are wide open providing little more than a minimum delta pressure for the application. If the compressors are adjusted to modulate, jacking up the pressure using the subtractive pilot or cash acme valve will simply reduce the capacity of the compressor while increasing the total systems demand and reducing the article pressures in the plant. The increased flow will also increase the delta pressure across the piping and supply components. The result is that your cost will rise, the supply will diminish, the demand willl artificially increase, and the point of use pressure may even drop. Now you are looking for another compressor or compressors and jacking up the supply even higher. We call this "Less for More". Not your best investment strategy.

So how does this all start? It begins with the air using OEM. It begins with how he selects components for the application, information from the air using componrnts OEM's, and rules for the use of air that will not take operating cost or energy efficiency into consideration. Most OEM's have a philosophy regarding accessory components such as hose, fittings, sealant, disconnects, lubricators, regulators, shut off valves, and filters. KISS!!! Keep the size small, keep it simple to operate, and most of all...keep the price down. See if the following is true for your point of use components selection:

  1. V1 tubing with push lok fittings.
  2. Double braided 125 psig hose with rubber core held on hose barb with water hose banding.
  3. Teflon tape for sealant.
  4. General line industrial disconnects generally compatible with Foster/Hanson style disconnect male and female fittings.
  5. Drip feed lubricators which work on a venturi principle. As you restrict the air across the lubricator, you increase the oil feed.
  6. Regulators, generally piston style.
  7. Filters- cintered bronze elements with a manual drain.
  8. Shut off valves - mostly gate or globe or non full port ball type.

 

These components are specified either by the tool crib or purchasing. There are no differentials specified such as differentail pressure at highest rate of flow at the inlet, in the case of tube and hose include maximum length, and lowest inlet pressure. In some cases it is also necessary to specify highest inlet temperature. You can also select components base on C's of V charts for the component, although most OEM's don't have this data. Catagory A is generally exprssed in terms of OD in fractions of an inch. On paper Catagory A & B will produce the lowest pressure drop at the highest pressure (when one asssumes that the flow remains constant regardless of the inlet psig). A, B, D, E, F, G, and H, generally speaking, are all the same size regardless of flow or psig requirement. Most of these components are selected based on either 1/4" or 1/2" size. When these components are selected on these criteria, the article pressure varies as a function of the flow and resulting differential pressure even when the inlet pressure is constant. This obviously effects the quality and integrity of the application.

I had the good fortune of direct reporting to Dr. E. Deming on a major automotive project in 1978. After our initial evaluation of the system, we met with engineering management for our first review. Dr. Deming described the operation of the system as "minus nothing-plus anything". By this he meant that the focus of the utilities management was on maintaining a minimum acceptable results for the system. They agreed. When we asked how high the pressure would pertubate to, no one knew. He discribed the results as poor quality results in manufacturing at highest possible cost. The chief engineer asked what he felt the alternative should be. Dr. Deming described it as "Plus nothing, minus something". In this scenario, as an example, the standard would be an article pressure of no more than 76 psig and a design differential of 5.2 psig at highest inlet flow and ternperature and lowest inlet pressure at specified rate of fllow plus <3 psid for dirt loading and the impact of leak attrition. The results are lowest operating cost at the highest possible quality. This is the essense of Statistical Process Control. This was not the end of the lesson. The engineer didn't like Demings analogies. Dr. Deming observed the engineers concern and said "what's the problem?". Before the engineer could speak, Dr. Deming said " I guess youn have an assignment of responsibility problem. When you operate based on plus anything, minus nothing, no one is responsible for the quality results in manufacturing, nor the cost of services." The engineer obviously didn 't line Deming. His retort was " OK, so let's say we agree to pllay your game.....only at 81psig instead of 76 psig maximum inlet pressure. Dr. Deming was profound at this point when he said "I suppose we could live with that. I would rather be consistantly wrong, rather than inconsistantly right.....at least we would know what the problem was." For me, this forever has changed the way that I have looked at air systems.