More Than One Valve Or Pump Choice for Every Application
Todd Loudin
President/CEO North American
Flowrox, Inc

With hundreds of different types of valves in the industry today and thousands of manufacturers globally how do you know which is the best product for your process? Chances are that there may be several good alternatives that could serve you well with a high mean time between failure (MTBF). But the solution is not always 100 percent clear. The reason there is not a one size fits all. Solution is that every process has variables. The variables can be numerous and certain types of valves will perform better in one set of variables than another. It is often not as simple as just the pressure, temperature and the medium.

Pressure, temperature and medium are excellent starting points though. But what about other variables?

Other factors that may impact your valveselection may be:
  • Specific gravity of medium
  • Percent solids
  • Vertical valve installation
  • Isolation required in both flow directions
  • Minimum, normal and maximum flow
  • Manual, pneumatic, electric or hydraulic
  • Supply air pressure provided
  • Size of valve
  • Threaded Connections
  • Velocity of flow
  • Percent solids
  • Horizontal valve installation
  • Isolation required in only one direction
  • Concentrations of chemicals contained
  • Required tightness of valve
  • Voltage of electricity provided
  • Pipeline material in which the valve be installed
  • Flanged connections
The valve manufacturer or representative who asks a high number of questions and requires specific information will probably be the ones that you should trust. By capturing as much information as possible hopefully leads to less headaches and troubles for the owner.

Acceptable MTBF varies from process owner to process owner and also in various applications. In some brutal processes, a valve that needs to be repaired every six months may be acceptable. In certain processes, if a valve fails or needs repair every 10 years then that valve can be considered poor quality or the wrong choice for the application. If your process is clean and non-corrosive you may choose from a wide variety of inexpensive ball valves. Even when there are corrosive chemicals, you still choose a ball valve with different alloys that are able to survive in corrosive chemicals. Ball valves are produced by the thousands and are widely utilized in a high number of industries.

Dealing with Abrasive Mediums
What choices do process owners have when they have an extremely abrasive slurry? The valve selection quickly becomes more limited. Of course the owner can decide to standardize and stick with a ball valve or similar. However, by doing so they are likely to face frequent repairs or replacements of that valve. If the slurry is 35 per cent solids, lime slurry , or 60 percent minerals slurry then the options for a valve with long MTBF becomes even more limited. For instance, lime slurry is unique slurry utilized in a multitude of industries such as chemical processing, mining, pulp and paper, power production, iron and steel, and many more. Lime causes many troubles for various types of valves because it combines the abrasive aspect of the slurry as well as the tendency to scale inside the pipeline and on stationary surfaces. Lime does not dissolve in solution but rather is a suspended solid. It is necessary to keep lime slurry in motion or the particles will settle out of solution and create more process problems. Valves that have areas for material accumulation will likely have problems dealing with lime slurry. For instance, with a ball valve, the metal ball will become coated with a build-up of scale and this scale will be dragged through the Teflon seats when the valve is opened or closed. This action will begin to destroy the Teflon seats and the valve's ability to continue to close tightly. Also the body cavity area of the ball valve will collect build-up that will force operating torques continuously higher. This can lead to the valve actuator stalling and ball valve stem breakage.

In slurries containing minerals of up to 60-80 percent solids, a standard valve will not be likely to survive the abrasive nature of the medium. So process owners might consider different alternatives than their standard valves. Two types of valves that are suited for abrasive slurries are rubberlined knife gate valves and pinch valves. Pinch valves utilize a rubber sleeve that lines the entire inside of the valve body. This rubber sleeve completely isolates the internal moving components during normal operation. In most On/Off applications the valve sleeve is 100 per cent full bore and has no areas for material accumulation. A pinch valve is also one of the only selfcleaning valves available. Any material build-up on the interior of the rubber sleeve is cleaned when the valve is closed. The rubber sleeve is flexed with each opening and closing thus breaking up scale and the build-up material is flushed downstream. Also, having all moving components of the valve isolated from the process medium insures smooth trouble-free operation.

Rubber-lined slurry knife gates also use rubber sleeves to combat extreme wear caused by abrasion. Many rubber-lined knife gates incorporate a flushing port on the bottom of the valve. This allows builtup material to be pushed out the bottom of the valve to allow for trouble-free operation. With scaling and highly sticky medium, the process owner might be wise to use a Tefloncoated knife gate to prevent and allow easy release of any scaling material on the gate. In a large majority of mineral-based slurries the Teflon-coated knife is not required. A secondary seal in the valve body helps to clean the gate of material build-up.

Another unique valve is a rotary disc valve. This product operates similar to a knife gate valve with a linear operation of the closing mechanism. However, instead of a metal gate used for closing, it utilizes a rotating disc. The disc rotates in a circular fashion on every opening and closing. The advantage is that the sealing surface is constantly being rotated so that the slurry does not continuously impact a single location every opening and closing. The disc secures tightly into a wedge upon closing and can incorporate rubber and polyurethane sealing in the seating area. This type of valve is suited for extremely high-pressure slurries up to 1,500 psig and higher temperatures.

Pumps like valves face the same types of challenges. Pump selection can be much more costly compared to valves if the incorrect pump is chosen. Pumps are dynamic and a working piece of machinery. If the pump is not running then the process is not flowing. Selection of a pump with a low MTBF may result in numerous process stoppages per year and costly maintenance and repairs. Similar to the above discussion of valves, which pump should you choose when dealing with abrasive and high per cent solid slurries?

There are multiple alternatives but let's examine a few well-known styles. The first highly-utilized type is centrifugal pump. They are very good for slurries up to approximately 40 percent solids. In most cases these pumps will have rubber -lined internals and rubber-lined impellers. Certain manufacturer's do in fact manufacture and sell pumps for slurries up to 50 -60 percent solids. However, process owners may learn that by pumping such thick slurries with this style of pump may increase repair frequency and repair costs quite substantially.

Progressive cavity pumps can be a viable alternative as well. This type of pump can vary substantially regarding the per cent solids it can handle and is greatly influenced by the nature of the medium being pumped. For instance , with a waste water sludge this type of pump can handle between 8-20 percent solids. However, if the medium is homogenous then it may be capable of up to 65 percent solids. One key downside to a progressive cavity pump is that it cannot be allowed to run dry or damage will occur to the rotor and stator. New designs incorporate evenwall stator technology and 2D and 3D rotor geometry. These newer designs can result in up to 17 percent greater hydraulic efficiency and more pressure produced per stage. The result is progressive cavity pump that runs at lower RPMs resulting in less wear and greater pressure produced in a smaller footprint. One other new advancement is the ability to change the mechanical seal without disassembling the rotor and stator or the motor and frame. This can save numerous maintenance hours and allow a mechanical seal to be removed and replaced in approximately one hour without removing the pump from its location.

The final pump product that is capable of very abrasive mediums up to 80 percent solids is the peristaltic pump. This style of pump can run dry for extended periods without damage to pump. By pumping 80 percent solids, a massive amount of water can be eliminated when trying to move mineral-based slurries. If a process owner moves a significant amount of minerals or slurries then the entire process plant can be downsized by pumping 80 percent solids versus 30 percent solids. A peristaltic pump also has the ability to perform suction lift applications. The nice feature of a peristaltic pump is that the only two normal wear components are the rubber hose and a small amount of glycerin in the housing.

Figure 4. Flowrox C Series PC pump with Even-Wall Stator and 3D Geometry Rotor and Removable Mechanical

It is important to mention that not all peristaltic pumps are similar. One style incorporates two metal shoes that compress the rubber hose. This style generates a significant amount of heat and requires a massive amount of glycerin in the pump housing to dissipate the heat that is generated. Also, this older shoe style design compresses the hose twice per revolution, which results in early hose failure.

Figure 5. Shoe design Peristaltic Pump.

New peristaltic designs incorporate a single roller that rolls over the rubber hose only once per revolution. This design eliminates the requirement for massive amounts of glycerin in the pump housing. Also, since the rubber hose is only compressed once every 360 degree revolution, the hose typically lasts 2-5 times longer than shoe design peristaltic pumps. The single roller design allows peristaltic pump availability to be significantly greater than old designs

When dealing with abrasive process mediums, users may have to invest more time and effort to find the best products to survive in these processes. The time spent trialing various products may reap some significant rewards. Process downtime and frequent costly repairs can be minimized. Certain types of valves and pumps can result in repair costs in excess of $100,000 per year. As technology advances there may be better alternatives that can eliminate or severely minimize these costs. When dealing with slurries of an abrasive nature, total cost of ownership (TCO) can have an extreme impact on the operating cost bottom line.