Uncommon Valve Specification Considerations - DeZURIK

Beyond system operating conditions (or &#;defined&#; operating conditions) can be other undefined expectations most often related to startup and shutdown procedures, upset conditions and emergencies. These lesser occurrences may be more severe than the actual defined operating conditions. Additionally, shutoff or control capability/performance and future facility planning may also be undefined expectations during valve selection. These expectations vary between control valves and isolation valves. The following categories are often overlooked, undefined expectations that may need to be considered.

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Valve specification and selection is an important part of every new system design. Many resources are spent identifying criteria such as service conditions and piping considerations. Over time, requirements can change due to capacity, product changes and operational variations. These variations often cause problems in the system that require a broader analysis to resolve.

Control Valves

Isolation capability

The PEC Eccentric Plug Valve was installed with the shaft horizontal so that the plug faced the flow when closed and rotated into the top of the body when open. Unlike the tapered plug valve, the plug in the PEC moves out of the flow path, allowing the green liquor to flow through the valve almost unobstructed when open. The valve internals are designed For isolating the green liquor from the slaker, the 12" handwheel moves the plug counterclockwise to the closed position. As the plug closes, it moves eccentrically toward the seat without scraping the seat or body walls, so there is no plug binding or wear. The rotating plug &#;cams&#; into the closed position and engages the integral body seat only during the last 5° of rotation.

Startup/Shutdown

Pump control is not typically addressed in specifying a control valve. It is common for startup and shutdown concerns to be addressed with a control scheme during initial system commissioning. Thus, the specification process usually does not account for short-term, potentially damaging, service conditions. As an example, a low- concentrate slurry may have little effect on standard control valve surfaces; however, during startup the control valve scheme may call for the valve to be slightly open to prevent piping damage from filling the system too quickly. During this interval, the slurry is eroding the internal surfaces, which degrades the control valve capability and life.

Many valve sizing programs allow for some flow conditions to be considered and predict damaging situations such as cavitation, flashing and noise. However, the startup or shutdown condition may fall out of the potential range capability of the valve selected to meet all other flow conditions. The assumption is that startup and shutdown are rare events and will not harm the valve. This erroneous assumption can cause startup and shutdown flow conditions to be removed from consideration, causing unexpected poor performance.

A rotary control valve with four interchangeable trim sizes allowed a mine to increase capacity and maintain accurate control without replacing the valve body.

Emergency Situations

Many control valves are equipped with a spring-diaphragm-style actuator that has a mechanical fail position. However, control valves can also utilize double-acting cylinders or electric actuation. These actuators will fail in &#;last position&#; upon air or power failure. Because terminology and understanding are not universal, it is important to clearly define what should happen during failure. For example, during power failure the valve positioner will lose signal, thus driving the output to fully open or close the valve depending on designation. Although the air compressor will also lose power, residual supply pressure in the system can continue to cycle the valve, causing an unwanted result.

Future Flow Requirements

Occasionally a control valve is sized for current conditions with the idea that it will also work for future planned expansions, alternate product runs, etc. This can end in unsatisfactory results as actual service conditions may be outside the ideal control range. A better solution is to specify a control valve with multiple internal, replaceable control components to accommodate the current application as well as a future one. Recently, a South American copper mine selected a rotary control valve that could be equipped with four interchangeable flow capacity ranges. This allowed the mine to install a common 2-inch rotary control valve body in its acid raffinate control on heap leach pads. When the mine was ready to increase flow capacity, it simply changed the valve seat, eliminating labor-intensive and expensive piping changes

Isolation Valves

Isolation Capability

Some organizations publish testing protocols and acceptable leakage rates for valves, including isolation valves. Valve manufacturers reference applicable standards in their documentation but typically do not publish the actual test criteria from the standard. Rather, they state whether the valve meets or exceeds the standard. It is up to the specifying engineer to understand the application criteria and apply the correct valve type based on the isolation capability requirement for the system. Additionally, manufacturers will often state shut-off capability in language that exceeds the minimum standard, such as zero leakage but again needs to be considered with their testing criteria.

A copper mine installed a two-inch rotary control valve body in its acid raffinate control on heap leach pads. When the mine was ready to increase flow capacity, it could simply change the valve seat, eliminating labor-intensive and expensive piping changes.

In addition to the common criteria listed on the application data sheet, a clear understanding of the isolation requirement must be identified. Does one of the testing standards clearly meet these application criteria? In the oil and gas industry, the API standard 598 is clearly understood and selection may be as easy as choosing a valve that meets that standard.

However, when considering a knife gate valve for isolation in pulp and paper, the decision may not be so clear. Under MSS-SP81, a metal seated, unidirectional knife gate valve is allowed to leak 40 milliliter/minute (ml/min) per inch of valve diameter with line pressure of 40 pounds per square inch (psi) against the gate. With an elastomer seated, unidirectional valve, the leakage rate must be specified or may refer to MSS-SP61, which has a lesser leakage rate. If the system has only minimal line pressure available to assist in seating the valve (less than 40 psi), then a bidirectional elastomer seated valve may be considered.

Requirements can vary greatly. For example, a metal-seated valve with an allowable leakage rate may be just fine with a 4% pulp stock slurry but would not be acceptable to isolate a sodium hypochlorite pump being maintained by a technician.

Emergency and Upset Conditions

Loss of power and emergency shutdown procedures affect isolation valves also. The same concerns need to be addressed as with control valves, except that positioners are replaced by solenoids and manual overrides may be needed for more specific and critical applications.

Upset Events

Upset conditions (including pressure, temperature and media abnormalities that may occur during an upset event) need to be reviewed. Surge protection may be required for pressure upsets. Elastomer selection may be affected by temperature spikes. Slurries may settle and plug piping, potentially causing a variety of issues from spills to dangerous exposure. Valve selection can be altered if a normal flowing slurry or bulk media needs to isolate under a static condition or column of media.

An example would be selecting an isolation valve between a bulk feeder and hopper. Under normal flowing conditions, a standard knife gate would slice through the flowing column and isolate. However, if the bulk feeder stalls and the column fills, the standard knife gate cannot cut through the column of material to adequately isolate the feeder from the hopper for maintenance. Other possible solutions could be an O-port knife gate or specialized, tapered body knife gate with space fabricated into the valve body to displace the column of material.

Specialized Isolation

Although common in the oil and gas industry, double block and bleed or double isolation and bleed valves are often specified for increased safety in all types of industries. These valves provide a detection port to verify acceptable shutoff from the isolation valves. Again, terminology, understanding and expectations can vary widely. To achieve the expected result, consult the manufacturer or its representative familiar with this style of valve and application.

Selecting the right valve for the application to achieve the desired result can be more involved than expected. Both current and future expectations must be considered and requirements communicated during the valve selection process.

Valves are an important component in aquarium filtration systems. They start, stop, and regulate the flow of water and prevent leaks in case of emergency or maintenance tasks. You&#;ll find valves used in refugiums, contactors, protein skimmers and on water pumps. Even canister filters use small shut-off valves on the quick-disconnect feature.

If you&#;re building or modifying a filtration system with valves, you&#;ll find a dizzying array to choose from online and in stores. However, not all valves are the same. Some valves are designed for regulating air flow or gases like propane. Using the wrong valve can reduce the flow rate and make it difficult to maintain your filtration system or even leach metal into your aquarium water. Here&#;s what you need to know about aquarium-safe valves and how to choose the right valve for your setup.

VALVES to AVOID

Let&#;s start off with the assumption that we&#;re discussing valves for controlling water flow in an aquarium. You can immediately eliminate the following types of valves from your shopping list. Most of these are found in big-box home stores.

• Brass ball valves
• Copper plumbing valves
• Gas valves
• Compressed air valves
• Aquarium air pump valves
• CO2 fine adjustment valves

There are a variety of reasons not to use these valves. Any metal can corrode in salt water and cause problems and gas valves are normally made of metal. Aquarium air line valves aren&#;t water-tight and often have metal parts. Some check valves have metal springs or hinges. Look for valves made of PVC. They&#;re completely safe and fully compatible with your PVC pipe and fittings. The two most-used valves in aquaculture, public aquariums, and hobbyist aquarium systems are ball, gate valves, and check valves. 

BALL VALVES

Ball valves have a spherical (ball) closure unit. The ball has a port (bore) through the center. When the valve handle is positioned so that the bore is in the same direction as the pipe, it&#;s in the open position, and water can flow through it. When the handle is rotated 90°, the ball turns and the bore becomes perpendicular to the flow path, closing the valve and stopping the flow. Ball valves come in a variety of sizes for almost any plumbing idea you can think of. They are typically used on the return water line to easily cut the flow of water but can also be used to regular flow to an extent.  

Example of ball valve positions from open to close

Types of Ball Valves - Ball valves come in a variety of sizes to match common PVC pipe sizes. The most basic and economical ball valve has slip fittings on both ends. The valve is glued onto the PVC pipe. Other options include the use of one or two true unions connections which make it easy to remove the valve for cleaning or temporarily remove hard piped equipment, like a return pump or reactor. Simply close the valve, loosen the union nut and the fitting can be removed.

Full Bore vs. Reduced Bore - A inside a reduced bore ball valve is one pipe size smaller than the valve's pipe size. This restriction causes a reduction in flow rate. The main purpose of a reduced bore ball valve is to throttle back the flow even when the valve is fully open. Full bore ball valves are more common in the aquarium hobby. The inside flow area of the valve matches the pipe size. If you&#;re shopping for a ball valve, just be aware of the difference and how it might affect your application. 

GATE VALVES

Gate valves have a flat closure panel that moves up and down instead of a rotating ball. The gate is connected to the stem of the handwheel. When you turn the handwheel, the gate moves up, allowing water to flow. Rotating the handwheel counterclockwise, lowers the gate back in place, stopping the water flow. These are best for your drain where regulating flow is desirable.

You may come across something called a knife or dump valve. They&#;re in the gate valve category but are made for applications where you want a fully open or closed valve without turning a wheel or lever. Just pull or push the handle to open or close the valve. It&#;s like an on/off switch for water.

CHECK VALVES

Check valves are designed to only allow water flow to go in one direction. The check valve is installed on the return line, between the return pump and return inlet. This prevents water from back siphoning when the aquarium return pump is shut off, which could otherwise cause a flood. When the pump is on, the water pressure pushes the valve open. When the pump is shut off, gravity will close the valve. Check valves are also used for auto-top-off plumbing, so that aquarium water does not back siphon into the RO or RO/DI reservoir.

CHOOSE the RIGHT VALVE for your AQUARIUM

Ball, gate, and check valves come in a variety of sizes and fitting configurations. While ball valves are the most popular style, gate valves are a bit more compact, in terms of the handle. The rotating handwheel fits in places where the lever on a ball valve may interfere with the filter system. Some aquarists like the precise flow control of a gate valve vs a ball valve. Where as check valves do not adjust the flow rate.

To choose the right valve note that PVC fittings are listed according to PVC pipe sizes, not the actual measurement of the part, and are categorized by the types of connections they make:

• MPT, or male pipe thread, has threads on the outside wall
• FPT, or female pipe thread, has threads on the inside wall
• PVC slip fitting, or slip fit, has no threads and glues to PVC pipe
• PVC insert fittings, or hose barb fittings, connect to aquarium tubing (not flex PVC pipe)

OTHER VALVE OPTIONS

Two Little Fishies offers a one-piece ball valve with barbed fittings which makes it easy for attachment to flexible tubing.

You might also encounter "quick release" valves which are common on canister filters and smaller diameter flexible tubing where a union is not practical.  These simply make disconnect easy and allow you to stop the flow of water before disconnecting.  

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