What are Valve Pressure Ratings?

04 Nov.,2024

 

What are Valve Pressure Ratings?

The sizing and selection of Control Valves have considerations beyond the flow capacity (Cv) relating to conditions they will encounter in operation. These are divided into two main ratings: Body Pressure and Close-Off pressure.

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Body Pressure Rating is a body and stem (shell) static rating, the amount of total pressure that the body and stem can contain without leaking. The body static pressure requirements for chilled and hot water valves are generally determined by the sum of the water column, fill, and pump pressures that the valve will experience. The pressure rating will be expressed in various ways depending on the valve manufacturer and type.

 

Common Ratings


WOG (Water, Oil, Gas) is a non-shock pressure rating for valves at ambient temperature. Oil refers to a free-flowing liquid containing higher viscosity than water and lubricating characteristics. Gas refers to a vaporized liquid, such as steam, and does not cover combustible gases. These will be expressed in PSI- 600 WOG equals 600 psi.

 

 

 

 

CWP (Cold Working Pressure) defines the maximum pressure rating for valves between -20F &#; 100F. These will be expressed in PSI- 200 CWP equals 200 psi.

 

 

 

PN (Pressure Nominal) is indicated in bar; for example, PN25 is 25 bar. 1 bar equals 14.5 psi. Sometimes manufacturers will include the psi rating as well.

 

 

Flanged or lug-style valves are categorized by ANSI flange classes covered under ASME B16 standards. Class 125 and 250 cover cast iron valves and flanges (B16.1). Class 150 and 300 covers ductile iron (B16.42), steel and stainless steel (B16.5). These ANSI classes have temperature pressure charts that correlate to the flange and/or valve ratings. Valves may indicate specific body pressure ratings exclusive of the flange rating.

Close-Off is the maximum differential across the ball or disc that the valve can hold against the rated seat leakage. The rated close-off pressure considers the entire assembly of the valve, actuator, and linkage. This pressure must be higher than the maximum differential pressure (Dp) the valve will be subjected to in the installation. The calculation is determined by the maximum pump differential for a two-way application. Valve close-off rating and maximum differential pressure rating may differ due to valve trim limitations. For example, a CCV may have a 200 psi close-off rating but a maximum Dp rating of 30 psi.

Download a detailed article on the various methods of calculating the required valve pressure ratings.

Valves - Choosing between Ductile Iron and Cast Iron

Customers from a wide range of industries require a variety of valves to meet their demanding processes. Valves are available in a large variety of materials for any conceivable application. Some of these materials are PVC, carbon steel, stainless steel, cast iron and ductile iron. In this article, we will focus on two of these materials: ductile iron and cast iron. The "ductile vs. cast iron valve" debate has been going on for decades, because not everyone knows the difference.

Fortunately, comparing valves made of ductile iron and cast iron is simple, as there are only a few major differences between these materials. We will outline the characteristics of both materials and then present the advantages and disadvantages of each. By the end of this blog post, you'll be an iron expert!

 

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Cast Iron Valve Specifications

Cast iron (also known as gray iron) is a metal that has been used for centuries. It is an alloy made of 96-98% iron, 2%-4% carbon and a small amount of silicone. It has impressive temperature tolerance, with some cast irons capable of handling temperatures in excess of F (C). In terms of pressure, the strength of cast iron depends on its pressure rating. The two most common are Grade 125 and Grade 250. At less extreme temperatures, Class 125 cast iron flanges are rated between 150 and 200 psi. Class 250 cast iron is tougher and is rated from 300 to 500 psi. these pressures may vary depending on the end type.

Cast iron is strong and usually does not break, even after strong vibration. The main disadvantage of cast iron valves is that they are not very ductile. Almost any bending will cause cast iron to crack and become useless. One of the major advantages of cast iron is that it is not very expensive. As far as metals go, cast iron is usually the most economical choice, which makes it a reliable choice for people on a budget.

 

Ductile Valve Specifications

The second material we are looking at is ductile iron. Ductile iron is a more modern iron alloy made of spheroidal graphite. This gives the material excellent ductility, so it won't necessarily break when bent. Temperature limits are slightly lower than cast iron, but still quite high at F (730C). In terms of pressure, ductile iron valves also use pressure grades: 150 and 300. 150 grade ductile iron maintains a seal of up to 250 psi at standard outdoor temperatures. 300 grade can withstand pressures up to 640 psi.

Ductile iron has excellent corrosion resistance, tensile strength and yield strength. Unlike cast iron, ductile iron does not fracture when bent, making it more suitable for demanding applications. Ductile iron is a strong and reliable material for pipes, fittings and valves. One drawback is the price. Because of its more complex chemical composition, ductile iron is usually more expensive than cast iron. However, if your application requires a tougher material, you may need to spend a little more.

 

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Ductile Iron Valves vs. Cast Iron Valves

Now that we have looked at these two options separately, we will compare and contrast ductile iron and cast iron valves. Hopefully this will help you decide which is best for you! We'll compare the price, durability and ease of installation of these materials so you know the most important details.

 

Price

&#; Cast Iron: Compared to ductile iron valves, cast iron valves will save you a lot of money due to their simpler manufacturing process. If you don't need the additional benefits that ductile iron offers, cast iron is the smart move.

&#; Ductile Iron: Ductile iron is the more expensive option, but for good reason. In many cases, it offers significantly better properties than cast iron.

 

Durability

&#; Cast Iron: Cast iron is a strong material that can withstand higher pressures than any plastic. However, when the pressure rating of a cast iron valve or pipe is exceeded, it may bend and crack. Cast iron has almost zero ductility, which means it is rigid and will break if bent. Cast iron will also rust over time, so regular maintenance is necessary to keep the valve free from corrosion.

&#; Ductile Iron: Ductile iron also has impressive structural integrity. It can handle high pressures and temperatures without retreating. Due to its ductility, this material is also less likely to crack when bent. Ductile iron will also rust over time, but not as easily as cast iron.

 

Easy to install

&#; Cast Iron: This material offers the user many options when it comes to installation. Cast iron valves are typically bolted using flanges. Welding is difficult, but not impossible. Our cast iron valves are available with flanged ends, as this is an excellent method for high or low pressure applications.

&#; Ductile Iron: Ductile iron offers the user many of the same installation options due to its similarity in construction to cast iron. Welding is not recommended, but bolted connections using flanges are still the best option for most applications.

 

There you have it! Now you know it's not a matter of "tough vs. cast iron valves" or anything like that. One isn't always better than the other. It all depends on what you are using it for!