Anti-scale valves provide improved performance, Solutions! Online Exclusives, January 2005, Vol. 88(1)

In process industries, valve applications offer many challenges: corrosion, abrasion, pressure shock waves, and temperature shocks, just to mention a few. Less frequently but often with more critical consequences, an effect is found in scale formation in valves. Another problem is dealing with sticky flow media. Both these effects usually lead to problems with the performance of valves and often also have a severe effect on the safety of personnel.

The consequences of these effects are often damage to the valve seats when the valves are actuated, which then leads to leakage through the valves. Deviation in the process, inconsistent quality of the produced material, loss of energy and increased production cost are the result.

The most critical consequence may be experienced when the valves cannot be operated at the required time. This can lead again to inconsistent quality of the produced material, possibly loss of a major amount of product and, even worse, possible personnel injury when the valves have safety issues. Clearly, affected valve applications must be analyzed, and solutions must be found, to minimize the effects of scaling and sticky materials.

Development of the O-Port
About 30 years ago, Stainless Valve Company first developed a valve called Big Blow for a pulp mill client experiencing severe problems using ball valves as blow valves on batch digesters. The life of the ball valves was between two weeks and two months. This unacceptable performance of the ball valves called for the development of a new valve that today usually has a service life between refurbishments of about 3 years or more, depending on the piping system.

Meanwhile, the same basic principle is used for our O-Port valves for other applications in many different industries, like power generation, food, oil and gas, petrochemical and others.

Developers learned much from the blow valves and applied the knowledge to the development of modern O-port valves for other applications. The basic principle of the valves is the same.

Figure 1: An exploded view of a 12" 900# valve

Figure 1 shows the valve body, consisting of the side plate 1 and 2, the blade (yellow) and the various sealing elements. The blade reaches through both ends of the valve body.

NOT a knife gate valve
One point in particular must be stressed. Often, the O-Port valves are thought to be knife gate valves. That is wrong; in fact the O-Port valves are in their function more similar to ball valves than to knife gate valves. The O-Port valves are also called through-port valves and slide gate valves. One major difference is that the blade reaches through both ends of the valve body.

At both ends of the valve body stuffing boxes, which we call packing glands, contain the blade guides. They are usually made of glass-filled Teflon. Their function is to properly position the blade inside the valve body and to support the blade against the forces generated by the pressure differential between the upstream and the downstream side of the valve.

In this way the function of the packing material is only to seal the valve inside to the outside and not to support the blade. This is very important for a long service life of a valve. The separation of guiding and sealing guarantees, that the packing material is not compressed axially by the pressure differential on one side, creating a leak path on the other side.

The double seats seal against the flow through the valve in both directions. The valves do not have a preferential flow direction.

The dark brown parts are called the pushers or followers. These parts apply pressure against the blade guide/packing material set. This force expands the packing material and in this way seals around the blade.

Development carried out by Stainless Valve Company and New England Braiding Company resulted in packing material, which forms well around the blade and has certain elasticity. This elasticity is important to accommodate the elastic deformation of the valve body and packing gland under pressure.

Figure 2 shows an 8" 150# valve which has some similarity to a knife gate valve, but again it is in its function completely different from a knife gate valve. This valve has the same 150# service (PN10) face-to-face dimension of a knife gate valve, but that is as far as the similarity goes. Unlike a knife gate valve, the blade passes through both ends of the valve body and is well guided at both ends in the packing glands. The blade has a hole in the middle, which is either lined up with the pipe or moved inside the valve body. The valve has spring loaded double seats for a class VI shut-off independent of the pressure differential across the valve. The sealing around the blade is the same as explained above.

Figure 2: An 8-inch 150# Stargate-O-Port® Valve.

AS-Stargate-O-Port valves
The special aspect of the AS-Stargate-O-Port® valve is the combination of coatings and seats in order to eliminate the negative aspects of scale formation or sticky flow media.

A variety of different coatings can be applied to the blade. We found the best coating to be used for low temperature applications to be the "high-build". It is a Teflon-based coating that creates a barrier between the blade and the flow media. The high-build has a good adhesion to the blade but the flow media (and therefore, the scale) does not stick to the high-build. This makes it possible for the seats to wipe the blade clean when the valve is actuated.

For high temperature applications mostly a nickel-based coating is used. Again, the desired property is a low coefficient of friction between the coating and the flow media.

Advantages
The O-Port valves offer a variety of advantages:

1. Proper actuation of the valve is guaranteed because no material is pushed into the bottom of the valves, which would prevent proper closing. (A knife gate valve or a butterfly valve cannot handle solids.)
2. The valves have a class VI shut-off, independent of the pressure differential between the upstream and downstream side. The tightness of the shut-off does not depend on a pressure differential between the upstream and downstream side.
3. The O-port valves do not need lubrication because there is no metal-to-metal contact between blade and body.
4. The valves can be used as shut-off valves, flow control valves and line blinds. Knife gate valves or butterfly valves cannot be used as line blinds, because there is always a force component opening the valves from the line pressure. In the case of ball valves, the outside indicators can show a closed position but in the inside the ball may be in a completely different position. The stem may be twisted, broken or there may be play between ball and stem.
5. Even in a partially open position the O-port valves have a straight flow direction, different from ball valves where the flow is re-directed twice.
6. The seats of O-port valves are not in the flow path. In case of a ball valve in a partial open position the flow hits directly the seats, causing premature wear.
7. Short face-to-face dimension, a fraction of the space requirement of ball valves in the flow direction. On the other hand, the O-port valves require more space perpendicular to the flow direction.
8. Foreign matter does not get jammed between the gate and the body in O-port valves, because there is only a very small space between blade and side plate. This prevents seat damage so that the valve always seats and seals well. Over 97% of the body thickness is solid material, resulting in a very stiff valve body. 3% or less are the gaps between the blade and the side plates.
9. The O-port valves guarantee long leak free operation, through the valve and from the valve body.
10. Pressure shock waves, aggressive flow media are well handled by the O-port valves.
11. The O-port valves can be installed in any position in the pipe; they do not have a preferential flow direction. Liquid, gas and solid flow media are handled easily by the O-port valves. The valves are the choice for handling solids in flow media.
12. Scale forming flow media or sticky substances can be handled easily by the AS version of the O-Port valves with the proper choice of coating and seat materials.

SVC Solution AS-Stargate-O-Port® Valve
SVC has developed special valves, based on the well known Stargate-O-Port® valves, for scale formation or sticky materials. Testing several combinations of coatings and seats in the lab at SVC, researchers found a combination that prevented the powder from sticking to the valve blade or seats. Preventing the accumulation of compacted powder layers on the blade and the seats gave the valve proper sealing characteristics. The valve was run for 15,000 cycles without showing any difference in performance. Because of the space constraints at the customer’s site the actuator was side mounted, which can be done because of the guiding system of the blade in the valve.

Figure 3: A 4" 1500# valve with side mounted actuator

Figure 4: An 8" 300# AS-Stargate-O-Port® valve

Application: Lime and fly ash storage
Lime storage bins have usually some sort of shut off valve, mostly those valves stick because of the hygroscopic nature of the lime. Often it takes hours of hard work with a sledgehammer and come-along to open or close those valves.

Because of an injury, an SVC customer had contacted SVC to find out about solutions for this problem. Technicians devised a concept and the valve was ordered and installed. It has worked since 1996 to the satisfaction of the customer, who meanwhile installed one more valve on another lime storage bin.

The same basic concept has also been used for fly ash. Fly ash from the boiler systems in the manufacturing is stored in bins, which are shut off in the bottom with an isolation valve. This valve allows the isolation of the rotary feeder valve/ screw feeder. The fly ash is fed through the rotary feeder and the screw feeder into a dump truck for disposal of the fly ashes.

The isolation valve has to deal with an abrasive material, which increases the internal friction in the valve. This does not allow a standard knife gate or others to function properly. Friction reduction and increased actuation force are necessary to have a well functioning valve for the isolation of the fly ash storage bins.

SVC has developed an AS-Stargate-O-Port valve, which meets these requirements. It is supplied with a hydraulic actuator and a hydraulic power pack. These valves are also used successfully in power generating plants.

Figure 5: A 4" 150# Fly ash valve for up to 1200 deg F

Application: Liquor heater valves
During the last eight years, Stainless Valve Co. has done some work with several customers on the application of our AS-Stargate O-Port® valves in liquor heater applications.

At any given time two of the heaters are heating the black liquor going to the continuous digester and the third heater is being acid cleaned. Typically once a week the heaters are switched. The previously cleaned liquor heater comes online and another of the two heaters is being cleaned. In these cycles every liquor heater is running for two weeks and then cleaned in the third week.

The main problem with liquor heater valves is scaling. When scaling happens on ball valve valves either the ball valve does not actuate at all after a short time or the scaling destroys the seats of the ball valve.

The AS-Stargate O-Port® valves feature a special coating to reduce friction and adhesion of scaling on the blade and a sufficiently large size actuator to guarantee proper actuation of the AS-Stargate O-Port® valve.

At the first customer’s site the AS-Stargate O-Port® valve, together with a competing ball valve, were installed in spring of 1995 side-by-side on two liquor heaters.

After one year in operation the customer decided to use the AS-Stargate O-Port® valves on all the liquor heater applications. Several more AS-Stargate O-Port® valves are meanwhile in operation successfully at this site. Because of recommendations from this mill, other mills in the same organization are using our Stargate O-Port® valves successfully for liquor heater applications.

The first liquor heater AS-Stargate O-Port® valve is in service at the second mill since about the beginning of 1997. Several more valves are in operation in other mills since beginning of 1998.

Figure 6: An 8" 300# valve body with 10" flanges

CONCLUSION
Stargate-O-Port valves have been developed to support use in demanding applications, where scale formation and sticky substances can prevent standard valves from performing properly. The anti-scale Stargate-O-Port valves have been used in various industries with remarkable results.

About the author:
Dr. Dirk Lindenbeck is president of Stainless Valve Co., Monroe, North Carolina, USA. Contact him by phone at 704-289-5858, or visit the company’s website: http://www.stainlessvalveco.com