In a variety of processes, there is a potential for a medium to change state, from a liquid to a gas. While this phenomenon is typically associated with high differential-pressure applications, some processes — such as caustics production and condensate return systems — are designed to promote flashing conditions in low-pressure systems. In such cases, care must be taken to ensure the control valves are capable of withstanding the damaging effects that can occur under these conditions.
|Figure 1. Depiction of the relationship between the vena contracta and vapor pressure|
For conventional valve geometries, this can be a difficult task. The typical flow path of most control valves directs the flow to a particular region of the valve body and trim, often resulting in some degree of material damage. Options exist to help counteract this issue. However, the user must always weigh the benefits of these options against the cost of implementing them. In many cases, the initial purchase costs outweigh the benefits of rectifying the situation, so the user becomes accustomed to regular maintenance of the valve. In cases where the efficiency of the valve is not limited and the maintenance requires inexpensive spare parts and falls into normal shutdown schedules, this may be acceptable.
On the other hand, if the damage is significant to the point that frequent maintenance is necessary to keep the valve in good operating condition and unexpected shutdowns occur, the efficiency is hindered to the point of unacceptability. At that point, the client will typically invest in upgraded valve materials and configurations, normally at substantially higher costs than that of the original valve.
|Figure 2. Schematic of chemical system where valve trim was being damaged by flashing service|
However, it may be possible to limit additional costs significantly, if the valve trim geometry is conducive for handling flashing service without being damaged and without compromising efficiency.
Such was the case when a major chemical company located in Rotterdam, Nether- lands changed from a rising-stem valve with the conventional S-shaped trim geometry to a valve with a direct, straight-through flow geometry. The following case study investigates this specific application and the problems the customer faced. Several of the options offered as solutions will be considered, as well as the valve design that the chemical company ultimately implemented in place of its failing S-shaped valve design.
Flashing Service Defined
|Figure 3. Diagram of cage-guided valve
Maintenance Log for
The following log was obtained directly from the user”s files, with notes made on the drawing. Those notes were part of an internal process that documented the history of the valve and any problems the user experienced. Taking information supplied directly from the user, the valve”s history, as it corresponds to the markings in Figure 3, was as follows:
19.04.1999 – Valve installed, flow in direction A
When a liquid passes through a valve and the resulting outlet pressure is below the upstream vapor pressure of that liquid, part of the liquid vaporizes. This change in state is known as flashing. As the medium passes through the vena-contracta of the valve, it is at that point where the pressure drop is the highest and where the medium begins to flash.
Unlike cavitation, the vapor remains in the downstream flow, as the outlet pressure does not recover above the vapor pressure of the liquid (Figure 1). This is why the severe damage typically associated with cavitating flow is often experienced in a flashing pipe.
Although damage due to cavitation is usually more severe and considered more troublesome than damage due to flashing, significant material erosion to valves and piping can still result from the high velocities and entrained moisture of vapor when it contacts process components. In many cases, this damage will be contained in the valve subassembly, particularly in the trim and on the wall of the valve body. Depend- ing on the flow path through the valve and whether or not the liquid is near saturation at the inlet of the valve, damage to both the valve trim and body can occur.
|Figure 4. Cage trim design typically employed for cavitation control|
The damage associated with flashing is often described as a very smooth, almost polished appearance. Over time, this results in the removal of material to the point that, if left unchecked, performance decreases. In the downstream piping, the moisture entrapped in the vapor can also damage piping. Elbows or fittings placed directly after the valve can be impacted as well and will eventually need replacement.
To counteract the effects of flashing service, a variety of options are available to the user. Hardened materials, expanded outlet piping, and unobstructed outlet flow paths can allow the user to manage the flashing medium. While these options may be significantly higher in cost than a standard trim design, they may be required to handle the flow.
At a production facility of the chemical company under consideration in this case study, the condensate from the steam process piping was being returned to
a condensate holding tank. From there it was further directed to a low-pressure flash tank for use elsewhere in the plant. The process conditions were:
• Medium: Condensate
• Temperature: 248 F (120 C)
• P1: 41.3 psia (2,85 bara)
• P2: 4.5 psia (0,31 bara)
• State: Flashing
• Inlet Pipe Diameter: 4” (100 mm)
• Outlet Pipe Diameter: 4” (100 mm), with downstream expansion to 14” (350 mm)
|Figure 5. Flow path of cavitation control cage|
A sketch of the process (Figure 2) shows the valve was situated between the condensate return tank and the downstream flash tank. The existence of flash steam was clearly imminent, so measures were taken to help control the erosive nature of the medium.
One measure incorporated by the client was the use of a deflector in the flash tank itself. This was situated across from the inlet of the tank where the steam vapor was directed. The purpose of this deflector plate, made in Monel, was to prevent the backside of the tank from being eroded by the high-velocity, erosive vapor. The pipe expansion was helpful in dissipating the flow over a larger area to prevent the medium from being concentrated to one particular area of the tank, exacerbating erosion.
|Figure 6. Characterized ball trim|
The valve in service employed a typical cage-guided trim. A cutaway drawing of the actual valve is shown in Figure 3. Based on the log of the valve shown in Figure 3, it”s obvious the customer expended a significant amount of energy maintaining this valve. Although the application, per se, was not a difficult one — low-pressure control — the fluid dynamics made for unexpected (and undesired) erosion problems. In addition to the lost production time and spare parts costs incurred in repairing the valve, there was also a safety and environmental concern due to the recurring leakage through the body housing. This was a problem the customer absolutely had to solve and without spending inordinate amounts of time, effort, and money.
In discussing various solutions with the customer, several types of trim designs were considered — all of which were available from the same valve manufacturer. These trim geometries each had characteristics, which would provide some degree of improvement over the existing valve design.
|Figure 7. Sliding gate trim|
The first trim considered was a multi-hole, cage-guided trim similar to those used in cavitation service. The idea was to provide a cage with many more holes in it, which are diametrically opposed and would diffuse the medium into smaller streams. Presumably, this would reduce the effects of the flash steam. An example of this trim and the flow geometry is shown in figures 4 and 5, respectively.
In Figure 4, the trim design provides many more holes to dissipate the flow than in the cage-guided design already being used. In liquid service, the flow typically comes in as shown in Figure 5, from the outside of the cage inwards, to allow the medium to collide from each side and hold the erosive flow away from any valve trim components.
For gas service, the flow would be in the opposite direction — flowing from inside the cage outwards. This is often done to reduce the high noise levels sometimes found in gas service. The problem with either flow path direction in this case was that the medium was still going to flash into a moisture-containing vapor. The pressure in the tank was below the vapor pressure of water at that temperature. So, with flow into the cage direction, the flash steam would be directed at the lower wall of the valve body. With a flow out of the cage path, the flash steam would likely damage the upper wall of the valve body.
|Figure 8. Two-inch MK75 valve|
Although the multi-hole trim would likely improve the situation to some degree, it was not seen as an adequate solution. A second trim design considered was a rotary, ball-type control valve. An example of this is shown in Figure 6. With this type of design the flow patch is much more linear.
The internal geometry does not have the typical S-shape associated with the globe or cage-type control valve. The design shown in Figure 6 features a characterized plate situated in front of a ball. This characterized ball valve, coupled with a pneumatic actuator and valve positioner, will enable the user to control the parameter as is possible with conventional rising stem valves. While these valves are typically supplied with soft seats to seal the flow when closed, such as Teflon or similar materials, metal seats for higher temperatures are available.
In reviewing this design with the customer, it was clear the direction of the flow would have a much lower tendency to damage the valve trim and body. However, understanding that flash steam was imminent, there was concern about the wall of the body when the valve was operating at partial stroke positions. The contour of the inner diameter of the ball would tend to guide the flow at an angle towards the side of the valve body, instead of straight downsteam. As a result, the user believed the high-velocity and liquid-filled vapor would still cause material damage to the valve. While a likely improvement over the cage-guided design in use, the user ultimately decided this option would still not create the long-term, maintenance-free solution desired.
The third trim geometry considered was another reciprocating stem design. The primary difference between this design and the one employed by the cage design was in the orientation of the variable orifice. Rather than using a large, single, open seat with a reciprocating cage or plug, this design featured a moving, slotted-disc situated in front of a stationary, slotted plate. This design is called the sliding gate. The flow through the seat set is perpendicular to that of the pipe, as opposed to being parallel to the pipe like in a typical cage or globe control valve. The result is a much more streamlined flow path (Figure 7).
|Figure 9. Cutaway view of wafer valve|
An obvious characteristic of this configuration is that the slots in the disc and plate will permit a truly straight-through flow, even at partial strokes. Figure 7 shows a typical threaded-connection, sliding-gate valve. In the flanged version, a similar body construction exists. The area of concern on this application, given the imminence of flash steam, was the reduction in flow path near the outlet connection. While the majority of the flow would be directed in the center of the valve due to the slot arrangement in the disc and plate, there was a risk of body corrosion. As well, the stem is located just past the vena contracta and would be subjected to the flash steam. As a result, another variation of the sliding-gate design was proposed.
In the wafer form, Figure 8, the body is extremely compact and the outlet portion of the orifice also serves as the outlet of the valve (Figure 9). This arrangement is remarkably similar to a slim-plate orifice. It is generally accepted that water flowing through such an orifice at saturated temperatures will flash only after it has passed through the plate (Driskell pg. 161). After discussing this design with the user, it was felt this would dramatically reduce valve body (and trim) erosion and therefore, this trim geometry was selected.
Details of the Selected Design
While trim geometry played a major role in the decision by the customer to use the sliding-gate trim over the other options considered, several related factors contributed to the selection as well. These factors, often applied in cavitation service, were:
• Directing the flow away from valve (pipe) surfaces — The wafer design ensured the body would not be in contact with the vapor and the straight-through flow path ensured the medium was directed to the center of the pipe, away from the pipe walls.
• Reducing the flow energy — By throttling flow through a series of slots in the disc and plate, the medium was spread into several smaller flow streams instead of one large flow stream, thereby helping to reduce the erosive energy of the medium.
• Utilizing wear-resistant materials — The sealing surfaces of the trim were coated with an extremely hard ceramic composite material called Jorcote. Converted to a Rockwell scale, this material has a hardness of approx 85Rc which, compared to various grades of Stellite, is significantly higher than other hardened trim materials.
|Figure 10. Wafer control valve|
A cutaway view (Figure 9) of the valve subassembly shows the multi-slot, straight-through flow path. Instead of flowing the medium through a single orifice, as is the case in a traditional rising-stem valve, this design has several slots. Depending on the CV selected, the number, size, and shape of the slots may change.
It is also clear from Figure 9 how the orifices are located in the center of the valve body, which is essentially exactly in the center of the outlet flange. Since the inner diameter of the flange connection is much larger than the diameter of the flow path through the slots, the medium is prevented from immediately contacting the outlet pipe wall and damaging the neighboring pipe and fittings.
Finally, the hardened seat coatings add wear resistance to the base stainless steel disc and plate. This hardened coating also has the benefit of a very low coefficient of friction (approx. 0.1) to allow the throttling elements to move smoothly during normal operation.
|Figure 11. Valve inlet|
While the design features of the sliding-gate valve made for a convincing argument, the real proof was in the actual performance compared to that of the original cage-guided valve. An actual picture of the valve installed is shown in Figure 10. This valve was supplied with an emergency handwheel and digital positioner as specified by the user. The green actuator was also a user requirement to indicate a normally closed valve configuration, recognizable by operators from a distance.
After installed, the user decided to remove the valve for inspection during the next plant shutdown. Approximately 10 months after installation, during which time the valve was in continuous operation, the valve was removed. To the technician”s delight, there was no visible damage to the valve body or trim area.
Figure 11 is a picture of the inlet side of the valve. The slightly darker look of the slotted disc is due to the Jorcote coating. From afar, it would seem there would be no reason to expect damage to the valve inlet in this case, since the condensate was near saturation at the valve inlet, material damage would have still been possible. There were signs of heat stresses, but no visible signs of wear.
|Figure 12. Valve outlet|
For the outlet side, Figure 12 shows the condition of the downstream area of the valve after having been in use for nearly 10 months. There are signs the valve has been in a hot environment (discoloration), but no visible signs of erosion or wear from the media.
The orifices do not display any scratches, burrs, or other damage. Based on the customer”s experience with this valve and the previous valve, it appeared the valve was in good working condition.
Downstream of the valve, however, was the Monel deflector plate. To recall, this plate was installed to protect the wall of the flash tank from erosion. The plate was also removed by the user for inspection. The condition of the plate was much different than that of the valve. Clear evidence of damage was immediately noticeable (Figure 13).
Approximately 10 millimeters of material had been worn away during the past months, indicating the flash steam was highly erosive as had been experienced previously. The Monel plate certainly performed its expected task, however, the extent of the damage was noteworthy, especially given the fact that the valve was in sufficiently good condition to be placed back into the pipeline without any maintenance or repair being performed, which is exactly what the user did.
|Figure 13. Monel deflector plate|
The yellow markings shown in Figure 13 indicate the area of damage from the flash steam. Keeping in mind this plate was made of an alloy material, located several meters downstream of the valve and after a significant expansion in the downstream pipe diameter, it was clear the medium had potential to damage any metal surface in its flow path. The fact that neither the valve nor the intermediate piping before the tank were damaged is a testament to merits of a streamlined flow path for this type of service. The user was so impressed with the performance and life of this valve trim design that similar cage valves throughout the flash steam system were replaced with the sliding gate wafer design.
The lesson learned from this case study was that it is vital for users to know the effects trim geometry have on the medium flowing through a valve and on the downstream piping. When faced with flashing or cavitating liquids, a common practice is to switch from the conventional globe or cage trim to a very expensive multi-stage or labyrinth-type trim design. For high-pressure drop applications or other severe service conditions, this is often the correct practice. However, for lower pressure systems (i.e. ANSI 300/PN40 and below), a truly straight-through flow geometry, like the sliding-gate design depicted here, can be sufficient to provide the user options to effectively manage the flashing situations encountered, both efficiently and economically.
Dell Grunenberg is a product manager for Jordan Valve, a division of Richards Industries. He is responsible for development, technology, and marketing of all Jordan Valve products, including regulators, control valves, and sanitary valves. Mr. Grunenberg has been with Richards Industries for nine years. His involvement with the project under consideration here included initial selection of the valve used, as well as plant visits after initial installation and evaluation period described in the article. Mr. Grunenberg can be reached at dgrunenberg@ richardsind.com or 513 533-5610.
For More Information: www.jordanvalve.com
1. Stiles, G.F., 1976, “Cavitation and Flashing Considerations,” ISA Handbook of Control Valves, 2nd Edition by Hutchinson, J.W., pages 206-211.
2. Bake. E.A., 1976, “Supplementary Notes on Cavitation and Flashing in Valves,” ISA Handbook of Control Valves, 2nd Edition by Hutchinson, J.W., pages 219-220.
3. Driskell, Les, 1983, Control-Valve Selection and Sizing.
4. Haines, Bradford, 2002, “Suitability of Trim for Cavitation, Part 1”, Valve Magazine, pages 39-42.
5. George, J.A. “Valves Enhance Control, Regulation”, InTech, pages 34-35.
6. Lange, Rainer, 2001, “Gleitschieberventile ideal für gro_es Delta-P Resistent gegen Kavitation,” Verfahrenstecknik.