The Importance of the Right Trim

For Optimal Performance, Choose the Right Trim

When shopping for a new car, most of us look for something that will serve our needs while offering the best performance. It should be the right size and shape (SUV, van, compact), have the right options (engine size, sound system, tires), and be able to hold up to whatever abuse we dish out (dirty feet or paws, spilled coffee or juice). We also know going into it that it will require regular maintenance (oil and fluid changes), and some parts will need to be replaced from time to time (belts and tires).

When selecting the right valve, the same general ideas apply. To have optimal performance, it has to be the right size and shape, have the right options or parts (whether it’s a ball, gate, or butterfly valve, check valve or pressure relief valve), and be able to tolerate the conditions of use (temperature, pressure, and the chemical nature of the fluid). And just like a car, valves require regular maintenance with some parts occasionally needing to be replaced due to wear from regular use.

Recently we shared an article about choices in materials that comprise valve bodies and trim. (If you missed that, you can read it here http://www.cpvmfg.com/blog/valve-materials-many/ ). But trim is unique. Trim warrants an article all to itself.

The internal, replaceable parts of a valve…the trim…need to perform without fail. They are the moving parts that make a valve a valve.

What you need to know

If you’ve ever gone through the process of selecting a valve, you know it’s not exactly like browsing through a catalog. For optimal performance, you’ll need to have the following information to make the best decision:

1. The fluid — the first thing you must know is what will be flowing through the valve. If it contains fibrous particles or solids, valves that obstruct the flow (such as a butterfly valve) should be avoided. Not only are solids and particles erosive, but they may clog the opening. On the other hand, viscous fluids can create drops in pressure. Butterfly or ball valves are good options for viscous media.
2. The valve size — at first it may seem like the size of the valve has nothing to do with the trim. But an oversized control valve would have to be operated with a low or narrow opening, making it unreliable and causing the valve seat and closure to sustain more erosion damage.
3. The valve pressure drop (ΔP) — the difference between the upstream and downstream pressure of the valve is important for two reasons. First, a large pressure drop puts stress on the valve stem, so it must be strong enough to withstand it.

Second, high-pressure drops lead to flashing and cavitation, which can be very damaging. Flashing occurs when the pressure within the restricted flow area of a valve drops below the vapor pressure of the liquid resulting in vaporization. The vaporized liquid releases a lot of energy, moving the remaining liquid particles at a high velocity and causing damage to the internal components of the valve.

Cavitation occurs when the pressure changes that led to flashing begin to return to normal. With cavitation, vapor bubbles collapse or implode, resulting in a release of energy or a shock wave. In a closed system, when there is no escape for that energy, the internal valve surfaces and components are gradually worn down becoming brittle and susceptible to breaking or fracturing. Cavitation produces a grainy appearance.

1. The fluid — the first thing you must know is what will be flowing through the valve. If it contains fibrous particles or solids, valves that obstruct the flow (such as a butterfly valve) should be avoided. Not only are solids and particles erosive, but they may clog the opening. On the other hand, viscous fluids can create drops in pressure. Butterfly or ball valves are good options for viscous media.
2. The valve size — at first it may seem like the size of the valve has nothing to do with the trim. But an oversized control valve would have to be operated with a low or narrow opening, making it unreliable and causing the valve seat and closure to sustain more erosion damage.
3. The valve pressure drop (ΔP) — the difference between the upstream and downstream pressure of the valve is important for two reasons. First, a large pressure drop puts stress on the valve stem, so it must be strong enough to withstand it.

Where sulfide stress cracking is a factor for oil and gas companies, ANSI/NACE MR0175/ISO 15156 should be consulted for material guidelines.

Trim materials

The list of possible materials for trim construction is extensive. Some of the most popular options for each part are listed here.

The stem is connected to the disc, which is what opens and closes the valve. The valve stem has to be able to withstand the force needed to move the disc, so it has to be very strong, but also resistant to corrosion and erosion. Metal alloys are usually a good choice for the stem.

The valve packing is the gasket that seals up where the stem and body meet. Packing is frequently made of graphite or Teflon (it’s not just for frying eggs). Graphite can endure temperatures up to around 1500 degrees F. Teflon doesn’t create much friction, so it’s slippery; however, its maximum temperature is only about 500 degrees F. There are a few different Teflon compounds, so temperature tolerance does vary.

The disc is crucial in determining the valve’s performance and seal. Discs are commonly made of reinforced forged steel.

The seat connects with the disc to form the seal that’s responsible for stopping the flow. Seats and seals can be soft or hard. The seating material in most soft-seated ball valves is usually Teflon. In order to increase its strength, high-temperature tolerance, and corrosion/erosion resistance, it’s sometimes combined with graphite or glass powder. Stellite is a popular erosion-resistant alloy that is also used in seating surfaces where high temperature is a factor. Stellite is corrosion resistant and extremely hard.

Industry-specific guidelines

All of our products at CPV are manufactured to meet industry quality standards. The American Petroleum Institute (API) has arranged numbered groupings of trim components for the oil and gas industry to guide them through the selection process. NACE (formerly the National Association of Corrosion Engineers), the American Society of Mechanical Engineers (AMSE), and the American National Standards Institute (ANSI) all have recommendations or standards for valves as well.

At CPV Manufacturing, we take pride in helping our customers make the best valve and trim choices for their project…quality over quantity.