Valves for Difficult Shut-off and Control

Doing the same thing repeatedly again and expecting a different outcome is the definition of insanity. In my career I have seen this scenario played out many times. The question may be, do you have problems with this valve quite frequently?  As time progresses, many engineers and maintenance teams continue to utilise the same types of valves that have had good lifetimes within their processes.

As a manufacturer of valves and having visited many customers, a “good lifetime” is a personal perception. Outsiders such as valve manufacturers may have specific knowledge to increase valve lifetime. When the answer is no, we just put a new one in every two months and throw away the old. A seemingly insane approach to the problem the answer may also be ‘that’s the way we have always done it.’

There are plenty of difficult mediums that cause problems for standard valves. One example is anything that is pneumatically conveyed. Even baby powder carried by a pneumatic conveying system is abrasive. Many traditional ball valves or standard rubber lined butterfly valves will not perform well in this scenario. These valves will likely have to be repaired or completely replaced quite frequently.

Any valve that has restrictions, or in the case of a butterfly valve a disc directly in the middle of the flow stream, are valves that will likely not survive well in the most abrasive mediums. The ideal valve in many abrasive flow streams should be full port, with limited pockets or cavities for material accumulation. The valve should generally be manufactured in a manner to better survive abrasion. Body cavity fillers in some valves are a method to increase valve lifetime.

There is still a pocket or cavity present, but it is minimised and some increases in valve lifetime can be achieved. There are two schools of thought on how to handle abrasive flows. One is to utilise a metal valve with hardened surfaces such as Stellite or other coatings to help the valve survive longer than carbon steel or stainless steel.

The other method of combatting abrasion is to go softer. Rubber has been a successful material for abrasive flow streams. In some processes the piping is rubber lined to protect the pipeline from frequent repairs or replacement.

Lime Slurry or Milk of Lime

Lime slurry, commonly referred to as milk of lime, is a relatively difficult medium for many conventional valves to survive within. Lime is added to water and does not dissolve in solution but rather is a suspended solid. For this reason, it is necessary to keep a lime slurry loop moving so that the lime does not settle to the bottom of the piping.

A certain velocity is required to keep the lime particles from settling out. Any pocket, cavity or void area is a natural collection point for the lime to settle into. Once the lime settles into this area it becomes a hardened mass. For this reason, valves such as ball valves, plug valves and butterfly valves all have areas that can create operational issues which results in frequent valve failure.

Many process plants have the desire to standardise on specific types of valves and in some cases add deflector cones or other apparatuses to protect certain types of valves. Attempting to utilise ball, plug or butterfly valves in lime slurry will typically result in constant valve replacements and costly repairs. The reason these valves have issues is because of the body cavity area. Lime can penetrate these areas when the valve is opened and closed.

Also, in the case of a butterfly valve, the bottom shaft is a potential trouble spot. Once lime has settled into these areas, it hardens and makes valve operation much more difficult. This can create operator fatigue or even stem failure due to over torque. Lime also has the tendency to scale on surfaces, making the valve more difficult to operate.

Soft wear parts such as the rubber sealing of the butterfly or the seating surface of the ball or plug valve are destroyed by the abrasive nature of the scaled lime on the metal surfaces. When the scale surfaces turn through the softer parts such as the seats, they are damaged and over time lead to significant valve leakage and erosion. Often the cost of the repair or replacement is insignificant compared to the manpower cost and loss of production.

Lime is utilised in a wide variety of industries. Pulp & paper mills, steel mills, coal fired power plants (flue gas desulphurisation process), chemical plants, glass production, food and beverage, mining operations and water and waste-water operations are all industries that utilise lime slurry.

One type of valve that has been very successful in lime slurry isolation and control is a high-quality pinch valve. Regardless of the chloride content, there is usually a rubber compound to handle the most difficult service.  A pinch valve utilises a full port rubber sleeve inside the valve to survive the abrasion in on/off service.

For control applications the inner rubber sleeve may be contoured to fit the optimal required Cv of the control system. With pinch valves the rubber sleeve is the only part of the valve that has contact with the medium. Scale may build up on the surface of the rubber, but the way a pinch valve operates make the valve generally the only self-cleaning valve for lime slurry.

High-quality pinch valves pinch the rubber sleeve from the top and bottom simultaneously and close the valve 100% bubble tight upon closure. As the rubber is starting to stretch when being closed, the rubber begins to flake the scaling lime on its surface. As the valve continues to close further, the medium velocity increases and helps to clean the inner surface of the rubber sleeve.

The abrasion capabilities of pinch valves and the self-cleaning aspect are the main reason that pinch valves are the ideal valve for lime slurry. A high-quality pinch valve for lime slurry modulating control, in most cases, out-survives most V ported ball valves and even ceramic valves. In some cases, I have heard from customers that utilise a V ported ball control valve and they need repair every 3 – 6 months, where as a pinch control valve may need repair only every 1 – 2 years.

In coal fired power plant FGD scrubbers, certain manual and on-off air actuated pinch valves may only require repair every 10 years compared to a ball, plug or butterfly valves that require repair or replacement twice a year. Please understand that not all pinch valves are created equal and not all provide this type of long service intervals. Due to the variability of rubber compounding, some pinch valves are quite poor in quality.

Keep in mind anyone with a pair of pliers and a rubber hose essentially has a pinch valve. But a high-quality highly engineered pinch valve will perform much better than lesser brands. If you have tried a pinch valve in the past and had unsatisfactory performance, don’t give up.

You just selected the wrong brand. Product variability and quality in pinch valves manufacturers have probably the greatest differences of any valve type in the industry today. To be successful you will probably pay more for the best, but in the long run you will save considerably on operational expenses.

Another type of valve that has been successfully used in lime slurry is a slurry knife gate valve. There are so many types of knife gate valves available in the industry today. There are face seal, wedge seal, uni-directional, bi-directional, etc…

The type of knife gate valve that will perform the best in lime slurry service is a bi-directional valve with two massive rubber sleeves on each side of the gate. It will be a gate valve that has a push through design. With push through designs the gate pushes down and separates the two rubber sleeves. As the gate reaches the bottom of the rubber sleeves it dispels any of the hardened solid materials out the bottom of the valve body. A small amount of liquid and solids are discharged on each closure.

Also, in the top of the valve there is a wiping polyurethane seal that helps to scrape of the scale that may have accumulated and hardened on the gate. Teflon and other coatings can be applied to the gate as well to make it more scale resistant or easier to rub off any scale that has occurred. Coal fired power plants have used this type of valve in their FGD scrubbers for 40 plus years.

When selecting knife gates for abrasive service it is wise to select one that is 100% full port. The rubber sleeves should be 100% the same inner diameter of the piping. If the rubber sleeves are slightly smaller than the inner piping diameter, then the edge of the sleeves are constantly bombarded by abrasive flow.  Generally, a pinch valve is a much better valve than any gate valve for lime slurry service, but due to cost and size the knife gate valve is often preferable.

Pumping Lime Slurry

Similar to the valves mentioned above, many types of pumps can be used to pump lime slurry. Some types of pumps are subject to lime scaling and when large particle break off they do extensive damage and create excessive repairs to the pump.  But for all of the same reasons above there are certain types of pumps that will be a better type of pump for lime slurry.

Depending on your required flow rate produced by one pump versus another you may be forced to certain styles of pumps. If you flow rate required is 440 gallons per minute or less, a peristaltic pump may be the best pump for your application. But make sure you utilise the manufacturers recommended hose size for optimum hose life. If you are pumping beyond 440 gallons per minute then a rubber lined centrifugal pump may be your best alternative.  

One specific concern when pumping slurries is it to keep a continuous velocity of the slurry so particles are less easy to settle out of suspension. Also a pump that does not seals is ideal in scaling slurries. A peristaltic pump does not have packing seals and also the hose is the only part that comes into contact with the medium. It is usually the only wear part in peristaltic pumps.  

Single Roller Designs versus Multiple Rollers or Multiple Shoes

Peristaltic pumps are excellent pumps for handling high solids slurries. Many lime slurry loops have approximately 30 – 35% solids and has a specific gravity of approximately 1.2 – 1.35. Due to all the reasons mentioned above about pinch valves, the peristaltic pump basically has most of the same properties as a pinch valve.

The peristaltic pump has a rubber hose that is compressed to push the slurry through the hose. There are no pockets, cavities or void areas in peristaltic pumps and the rubber hose is excellent against abrasion.

Peristaltic pumps are also excellent for us in suction lift duties and can also run dry without any damage to the pump or hose. Most other types of pumps cannot run dry without causing pump damage. With peristaltic pumps you should focus on acquiring an eccentric single roller design. This design will compress the rubber hose only once per every 360-degree revolution. This is important because hose life much depends on how many times you compress the rubber hose.

A pump that compresses the rubber hose twice or more times per every 360-degree revolution will have typically ½ the hose life of an eccentric single roller design. Also, there are designs that also utilise two shoes or metal bumps to compress the hose. These designs generate a lot of friction and heat on the rubber hose. Adding significant rubbing action and heat to the rubber hose further deteriorates the rubber and leads to much shorter hose life. Two shoe pumps that create significant friction and heat require increased glycerin in the housing to dissipate the heat.

One power plant in Ohio pumping lime slurry was utilising a two shoe design peristaltic pump and complained their hose life was only approximately 518,000 revolutions before the hose broke down and failed. This power plant shared that they were spending more than $29,000 annually per pump for repairs and maintenance costs.

They replaced a 4” double shoe design with a 2.5” eccentric single roller design.  The hose lifetime increased more than 5 times longer to more than 2.7 million revolutions. Their maintenance cost was reduced by 69% by utilising the eccentric single roller design peristaltic pump. If you currently own peristaltic pumps and are unhappy with hose life and pump utilisation rate, then you might investigate an eccentric roller design peristaltic pump.

When you have flow rates exceeding 440 gallons per minute then you may consider a slurry centrifugal design. The impeller and throat, housing and discharge are rubber lined for the abrasion these pumps will experience. These type of centrifugal slurry pumps were designed for mining slurries with large particles and solids. This pump will serve you well. They are available up to 20” pump diameters and even larger in some brands.

Conclusions

Every medium is unique and for this reason certain styles of valves or pumps may be better for certain mediums. Lime slurry is just one medium that may cause issues not found in traditional valves and pumps. The above mentioned, valves and pumps are excellent for handling very abrasive high solids slurries.

Pinch valves and peristaltic pumps are capable at handling very high solid slurries even up to 80% solids. If you have high solid slurries, then you should think pinch valves and peristaltic or centrifugal pumps.

Anything that is pneumatically conveyed can effectively be isolated by a pinch valve. In pneumatic conveying a small damage and small amount of leakage in a valve will result in rapid deterioration of that valve’s shut-off capabilities. A pinch valve is bubble tight and much more resistant to this rapid deterioration. Lime scales, settles into pockets and voids and solidifies, causing operational difficulties in many valves and pumps.

The full port rubber sleeve or straight hose of a peristaltic pump results in self-cleaning of the rubbers hose or sleeve. The lack of pockets or cavities for material accumulation are two of the main reasons pinch valves and peristaltic pumps are so ideal for lime slurry handling.