Pumps For Chemical Industry
Advancements in overall efficiency an the ability to handle a wide array of unique chemicals combine to make the air operated double-diaphragm pump a first choice for chemical processors
The circumstances that prompted Jim Wilden to develop the air-operated double-diaphragm (AODD) pumping principle six decades ago have taken on almost mythic status: a ruptured water pipe, a flooded workshop and an exclamation from a coworker that “Slim” (Wilden’s nickname) could “make a million dollars” if he could invent a solution.
In the ensuing 60-plus years, those words have proven to be prophetic as the AODD pump technology that was said to be “conceived out of necessity, born in the arms of innovation, and inspired by sheer will and determination” has become a go-to choice for operators in rough-and-tumble industries like mining and heavy construction who require a pump that can easily and reliably pump water, slurry or any finely divided substance, such as cement, in diverse operating environments.
In many ways since their invention, however, AODD pumps have become a victim of their own success as their design characteristics allow them to obtain dry self-prime, run dry, maintain suction lift up to 30 feet (9 meters or 14.7 psia), withstand deadhead pumping conditions, operate while completely submerged and pass solids up to 3 inches (76 mm) in diameter.
This has caused many pump users to pigeonhole them as only useable in utilitarian, auxiliary or basic liquid-handling and transfer applications.
In reality, thanks to a series of technological advancements from Wilden® Pump & Engineering Company, that have further economized the AODD pump’s method of operation, the unit can now be considered a true “process” pump and has gained acceptance as such in diverse industries like paint and coatings, ceramics, adhesives and sealants, oil and gas, food and beverage, pharmaceutical and cosmetics, as the following examples help to illustrate:
• When one of the largest independent chemical distributors in the United States relocated to a new, larger facility with expanded warehousing, blending, packaging, clean room and warm room areas it needed a pump technology that could perform well in a number of unique operating environments.
The solution was to outfit the facility with bolted AODD pumps, which possess the capability to handle a plethora of different chemicals that would need to pass safely through the facility. “We wanted better product containment and the AODD pump has been fool-proof and has given us great performance with no worries,” said the distributor’s facilities maintenance manager.
A leader in solvent production for use in the Indonesian paint-and-coatings market was looking to improve the operational efficiencies at its production plants, where the time needed to load and unload holding tanks was becoming prohibitive.
After performing a series of onsite tests, the plant’s operators learned that advanced AODD pump air motor technology could not only reduce the transfer times through higher flow rates, but could do so with decreased compressed air consumption compared to competitive products, thus reducing the operating costs of the fluid transfer.
• A Spanish company that produces printing inks was having problems with the gear pumps that were being used in the production of water, solvent and UV-based inks for flexography and digital printing systems.
Namely, the gear pumps were very inefficient in their operation and constant
breakdowns were leading to increased downtime and maintenance costs.
The solution was found in replacing the ill-performing gear pumps with AODD pump technology featuring an advanced air engine that requires less air while still delivering the same flow rates.
“Compressed air as an energy source is relatively expensive, so if we can do the same work with half the energy it is very important,” said the company’s president.
“For me, the AODD pump is a very good choice and I am sure the results will demonstrate that.”
While these examples may show that AODD pumps can be considered process pumps in critical applications across a wide variety of industries, this has only become possible once some noteworthy refinements in their operation were made.
The Air In There
Despite the fact that AODD pumps have, from day one, proven their effectiveness in utilitarian liquid-transfer applications, there has always been one annoying glitch in their operation: at the end of every pump stroke, a small, but still significant, amount of air was wasted.
This kept the pump from operating at its most efficient and added to its bottom line cost of operation.
Because of that, AODD pump manufacturers – led, of course, by Jim Wilden – were always searching for ways to decrease or eliminate the air loss at the end of the pump stroke.
This has led to a series of advancements in Air Distribution System (ADS) technology that have enabled the AODD pump to optimize air usage (and cost) while still maintaining its standard-setting operational characteristics.
While air loss has been a constant concern within the operational window of AODD pumps, the earliest ADSs were designed first and foremost to battle another operational irregularity in the AODD pump’s performance: stalling and icing.
It was only once those performance inhibitors were conquered that designers turned their attention to developing ways to more efficiently govern the pump’s air consumption.
One of the first ADSs designed to promote energy efficiency featured a dial that could be used to tune the pump’s operating speed by restricting the amount of air that was allowed to enter the pump.
It’s a fact of AODD pump life that a slower running pump is more efficient. For example, a dial-in ADS running at full throttle may consume 50 standard cubic feet per minute (scfm) of air in order to pump 100 gallons per minute (gpm) of fluid.
Using the dial, the incoming compressed air can be dialed back to a 35-scfm rate, where the pump will transfer the liquid at a flow rate of 80 gpm. This is a 20% reduction in flow rate, accompanied by a 30% reduction in air consumption, which makes the pump more efficient.
While the dial-in ADS represented an undoubted advance in AODD pump operation, there was still more field that needed to be plowed if pump performance was to hit the sweet spot in optimized air consumption. That day came in 2013 with the development of a new ADS technology that featured a revolutionary air control spool, which is shaped, more or less, like an hourglass.
This innovation was driven by an evaluation of the pressure dynamics that occur within the AODD pump during its operation.
This evaluation clearly revealed that air consumption was significantly impacted by an increase in air pressure at the end of each diaphragm stroke. Specifically, when the shaft would come to a full stop at the end of each stroke a shift signal would be sent to indicate that the flow of air should cease.
However, there was a small time lag between the stopping of the shaft and the sending of the signal, meaning that the full force of the compressor continued to push compressed air into the air chamber, but that air was not doing any actual work and was lost to the atmosphere upon exhaust.
The function of the air control spool reduces the amount of air that is allowed into the pump at the end of the stroke, which drastically reduces the amount of “wasted” energy that had traditionally been “force fed” into the pump.
This allows the AODD pump to experience up to 60% savings in air consumption, while delivering more yield per scfm than AODD pump models that feature legacy ADSs.
Through Thick And Thin
The favorable reputation of AODD pumps is built on the technology’s versatility, or ability to handle a wide range of liquids with varying characteristics.
One of the most important is viscosity, or the thickness of the liquid that is being transferred, since the pump’s true best operational efficiency is only achieved if the lowest possible amount of scfms of air are consumed while delivering the highest possible flow rate, no matter the viscosity.
Specifically, in the operation of AODD pumps, slip is almost eliminated, regardless of the viscosity of the fluid, as it is controlled by the ball check valves in the pump.
Other technologies such as gear, screw and lobe pumps have fixed tolerances to control slip, which makes them unable to adequately adjust to viscosity changes.
When working with a centrifugal pump, the fluid viscosity is a design factor in the pump equipment and its selection for a specific application.
Impeller dimensions and styles are specifically tailored to be compatible with thick or thin fluids, making moving a pump from one application to the other not really workable.
Working with an AODD pump, however, the viscosity need not be a factor for operation.
This is the real benefit of the AODD pump in this realm – its ability to handle multiple or different viscosities without regard for equipment setup.
For example, gear pumps may be a good choice for very thick oil or viscous liquids, but they are poor choices for thinner liquids like ethanol or water.
The AODD pump, on the other hand, is a jack-of-all-trades that doesn’t care if the fluid is thick, thin, particulate-free or laden with particulates; its design allows it to pull in the liquid, no matter its composition, and drive it downstream.
AODD pump manufacturers do publish viscosity correction tables, but these are simply helpful in predicting pump performance given specific system discharge and air inlet pressures and a known viscosity.
In reality, the design of the AODD pump does not need to change for liquids that have highly viscous or very thin consistencies.
AODD pump manufacturers are also taking steps to improve the flow path of their pumps, which will make it even easier to efficiently transfer highly viscous or particulate-laden liquids.
In fact, some newer AODD pump models with redesigned and optimized wetted paths deliver flow rates up to 50% higher than legacy models.
This enhanced flow capability may also allow the operator to use, for example, a 2-inch pump where a 3-inch model may have been the choice in the past, with attendant initial cost of ownership and maintenance cost reductions – smaller pumps, smaller price.
A caveat must be noted, though, that the AODD pump does have a few performance limitations, most commonly on the suction side of the pump, since the only pressure that is available to bring the liquid to the pump is atmospheric.
In cases of extreme suction conditions, the physical location of the pump relative to the pumped media is a crucial component of application success, i.e. the pump should be moved as close to the source of the fluid as possible. This will reduce the line friction leading to the pump.
Whenever possible, locate the pump below the supply tank – this will enable gravity to assist the “feeding” of the pump.
Also, eliminate as many fittings and elbows on the suction side of the pump as possible.
Operators and system designers may additionally consider over-sizing the plumbing leading to the pump, all in an effort to reduce friction on the suction side.
These are some of the tricks of the trade that can be used to take advantage of the AODD pump’s versatility and ability to optimize air consumption and liquid flow rates.
There’s no question that the improvements in overall AODD pump operation and ADS capabilities have been significant over the years, and the same can be said for diaphragm materials and design.
As AODD pumps have begun to be used in more process-type applications the advances in diaphragm performance have kept pace.
Correct diaphragm material selection is critical to ensure safe AODD pump operation, with six primary factors to consider when choosing a diaphragm: chemical compatibility, temperature range, abrasion resistance, flex life, performance and cost.
To help meet these diverse operational criteria, the number of effective diaphragm materials has also grown and now consists of three basic subsets:
Synthetic rubber (Neoprene, Buna-N, EPDM and Viton®), thermoplastic elastomers (TPE) (polyurethane, Santoprene®, Hytrel® and Geolast®) and Polytetrafluoroethylene (PTFE or Teflon®).
Manufacturers have experts on hand to help pump users select the best diaphragm material and design for their applications.
Advances have also been made in diaphragm design that help make the AODD pump more hygienic in its operation, which allows its use in contamination-sensitive manufacturing applications like food and beverage, pharmaceutical and personal care.
Integral piston diaphragm designs place the shaft connecting plates within the diaphragm itself, which means that all product-entrapment areas and leak points between the piston and diaphragm have been eliminated, resulting in a reduction in the chance that product contamination or leaks can occur.
Integral piston diaphragms are also easily cleanable and there is no interaction between the diaphragm and the outer piston plate that can lead to abrasive diaphragm failure.
For the chemical facility operator, downtime and pump maintenance are typically more expensive than the spare parts required to keep the AODD pump operating.
This allows the pump to run twice as long before requiring maintenance and will translate directly to a healthier bottom line.
In general, standard AODD pump builds that feature PTFE diaphragms have a Neoprene backup for reduced-stroke configurations and a Santoprene backup in full-stroke configurations.
Santoprene is actually an excellent backup choice for both reduced- and full-stroke diaphragm configurations since it has excellent chemical-resistance properties and long flex life.
Another option is Hytrel® backup diaphragms; this material has the lowest compression set of any elastomer in use and performs well in sealing the diaphragm at the inner/outer piston interface and the outside diameter bead.
So, what does all this mean for the chemical-processing industry? By definition, chemical manufacture features some of the most intricate and complex industrial processes in the world.
The complexity of chemical manufacture is highlighted by the number of so-called “unit operations” that must be completed during the overall manufacturing process.
One of the most critical of these unit operations is the transfer of liquids along the production chain. Because of the importance of the myriad transferring operations within the entire chemical- manufacturing process – raw materials to storage tanks, raw materials to blending tanks, finished products into fixed-weight containers, etc. – facility operators need to identify the best pumping technology for the job, one that possesses the versatility to perform reliably and efficiently at any number of points in the production hierarchy.
It had almost become a rote choice among chemical manufacturers that centrifugal pumps were the best technology for transfer operations within the chemical plant, for several reasons:
• Centrifugal pumps work best with thin, water-like fluids, which have long been a staple in chemical manufacturing
• A kind of “if it ain’t broke don’t fix it” attitude has made the centrifugal pump an easy fallback option for operators who have undoubtedly worked with the technology at some point in their careers
• There was an overriding perception that centrifugal pumps have a lower operating cost when compared to the operations of an AODD pump, but this has been shown to not necessarily be the case.
A closer look, though, shows that despite its reputation, the centrifugal pump doesn’t appear to be the all-conquering technology that is required for efficient and optimized chemical- processing applications.
Specifically, centrifugal pumps work best when they are operated at their Best Efficiency Point (BEP). Unfortunately, that BEP is rarely realized for an extended period of time during fluid-transfer operations, which results in flow rates that can fluctuate constantly.
Additionally, consistent operation off the BEP can lead to potential problems, not only from the equipment’s operational point of view, but also in regard to the production process and the way the chemical is formulated.
Also, when a centrifugal pump operates to the left of its BEP, radial loads increase due to the way the pump generates pressure along its volute by reducing the fluid velocity.
This method of operation increases shaft deflection at the seal faces, which results in increased seal wear and a decrease in the pump’s life expectancy.
Working to the left of the curve will also increase axial loads that can overload the thrust bearings, especially in open-impeller and diffuser-type multi-stage centrifugal pumps.
Finally, as a centrifugal pump operates close to the zero-flow point (zero efficiency), heat will be generated at levels that can be highly harmful to heat-sensitive chemicals or products themselves, which can also adversely affect safety and the quality of the resulting product.
At the other end of the spectrum, when a centrifugal pump works to the right of BEP other problems can be created. Specifically, the level of net positive suction head (NPSH) required increases, which may cause efficiency-harming cavitation to occur.
Since the liquid-transfer process in the chemical industry, particularly when handling specialty chemicals, is managed in batches, an insufficient NPSH condition may be more complicated to detect, but it will deteriorate the pump’s operational capabilities continuously, meaning that the pump’s ability to handle any cavitation that occurs will be compromised.
By comparison, the fluid-delivery curve for an AODD pump is very similar to that of the centrifugal unit, without the negative behavior that occurs when running off the BEP.
The AODD pump will perform based on the inlet air pressure supplied to the pump and the system pressure it encounters. If adequate suction pressure is available, fluid will flow into the pump and be discharged based on the relationship between the air pressure operating the pump and the system pressure.
The larger this pressure differential is, the faster the pump will operate, and when the differential is reduced, the pump’s operation will slow.
This is referred to as “infinitely variable speed operation.” If the system pressure should increase unexpectedly, the pump will operate until the inlet air and system pressure are equal, at that time the AODD pump will stop – in what is termed a “deadhead” condition – with no equipment damage.
The system remains pressurized, but no differential pressure exists to continue driving fluid.
The pump will restart when the system pressure falls below the operating air pressure, which is particularly important in batch processes.
In contrast, a closed valve can wreak havoc on a centrifugal pump, and with different positive displacement pump technologies operating against a closed valve can cause damage or destroy a system by reaching the burst pressure of the plumbing in very short order.
If a valve is restricted on the inlet side of a centrifugal pump, the pump can cavitate, leading to impeller and volute damage.
If the inlet were closed completely, the operator should expect bearing and wear-ring failure due to heat buildup.
If this condition occurs when operating an AODD pump, the pump will slow and if the