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Last updateFri, 24 Mar 2017 2pm

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Valve Selection in Pulp and Paper Operations

Valve Selection in Pulp and Paper Operations

Over the centuries, the pulp and paper i...

How to Improve Control Valve Performance with Positioners

How to Improve Control Valve Performance with Positioners

As the final control element in most pro...

VMA Technical Event: Engineering Valves in the Extreme

VMA Technical Event: Engineering Valves in the Extreme

While much the 2017 VMA Technical Semina...

FEL: The Preferred Phase for Valve Technical Definition

FEL: The Preferred Phase for Valve Technical Definition

A typical oil and gas capital project cy...

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Industry Headlines

Kansas City Fed Shows Manufacturing At 6-Year High

Friday, 24 March 2017  |  Chris Guy

Tenth District manufacturing activity strengthened further in March, and many indexes of expectations for future activity were at or near record highs...

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Industry Headlines

GE Oil & Gas Expands with New Sub Saharan Africa Facility

2 DAYS AGO

GE Oil & Gas opened a new facility in Takoradi Port, Ghana, expanding its global footprint and supporting local investment. The facility, which will be the primary service center for deep-water offshore projects in Ghana, has a 1,600 square meter indoor test area with capability for testing thre...

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Metso Launches Digital Program to Accelerate Growth

3 DAYS AGO

Metso has set up a Digital Program to accelerate the company’s digital capabilities, which are required to succeed in the future of minerals processing and flow control. Metso's ambition is to become one of the digital leaders in the industries it serves. This means for example turning custome...

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Shell Looking to Lower Cost of Offshore Projects

2 DAYS AGO

“With oil prices hovering around $50 a barrel or less, and a rising amount of renewable energy chipping away at market share, Shell is going lean on its deepwater projects to make sure it can eke out a profit from all of its operations,” writes ChemInfo .

For example, Shell is attempting to ...

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Report Says PA Can Support Four More Ethane Crackers

2 DAYS AGO

A new report from IHS Markit, commissioned by the state of Pennsylvania, forecasts $2.7 to 3.7 billion in investments in natural gas liquid (NGL) assets as well as the opportunity to attract additional cracker plants, and petrochemical and plastics manufacturing. IHS Markit believes the Marcellus an...

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Kansas City Fed Shows Manufacturing At 6-Year High

1 DAY AGO

Tenth District manufacturing activity strengthened further in March, and many indexes of expectations for future activity were at or near record highs. Most price indexes increased moderately. The month-over-month composite index was 20 in March , its highest reading since March 2011, up from 14 in Fe...

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Consumer Sentiment Reaches Highest Level Since 2000

5 DAYS AGO

The overall level of consumer sentiment remained quite favorable in early March due to renewed strength in current economic conditions as well as the extraordinary influence of partisanship on economic prospects. The current economic conditions component reached its highest level since 2000, largely...

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Cast or Forged?

materials_q_and_a_graphicQ: Should I be concerned whether my valve is cast or forged?

 

A: Just as in politics, everyone has an opinion on this issue.

 

The good news is that both types of valves should be able to provide you with acceptable performance, although a perception exists that forged valves are superior to cast valves. However, if cast valves are made properly, they can and have worked equally well in a variety of services and usually at a much lower cost than forged valves. Also, the belief that forged components are infallible is not true. Let’s look at an example of a 4-inch diameter wrought valve stem in N07718 (Figure 1). A crack-like defect was seen on the end of the stem, so the part was cut in half, and the large shrink cavity you see here was discovered. This shows that forged material is not without its own problems. But let’s examine how cast and forged valves are made and how we can assure we get a good valve.

What most people don’t realize is that cast and forged valves start out the same way—molten metal is poured into a mold or ingot. As a result, both types can have defects such as the shrinkage in the above mentioned N07718 bar. Other defects associated with forgings are inclusions, laps, seams, cold shuts and cracks. Defects with common castings are inclusions, porosity, misrun and hot tears. As you can see, both have their potential issues.

An issue with forgings often overlooked is that forgings and wrought products will have non-uniform mechanical properties. This is because they are worked or formed more in one direction than in another. Therefore, the grains will be elongated more in one direction than in the other, which has a direct affect on mechanical properties, particularly impact strength. As a result, the design of forgings needs to take into account these anisotropic properties whereas castings have uniform properties no matter what the orientation of the test coupons.

Another advantage of cast valves is that they can be produced in more complex designs than forged valves. Certain valve designs such as a globe valve are simply difficult or impossible to produce as forgings. This flexibility of design in cast valves allows them to be more efficient in controlling flow than a similarly forged valve.

Something else to consider with forged valves is that they usually are made in halves, particularly the larger sizes. This means there is either an additional flanged connection that can be a potential leak path or the halves are welded together. Welding, however, is another process for cast metal that can have its own set of problems.

The questionable reputation that castings have is from two sources. First, most of the ASTM cast specifications are lenient in requirements for composition, heat treatment and inspection. Second, some foundries either use this latitude to their advantage or simply do not know enough to implement tighter controls when needed on chemistry or heat treatment. This concern about castings has resulted in equipment produced to ASME Section VIII having a quality factor on castings of 80% of the allowable stress values for a wrought component. However, this quality factor can be increased to 100% if sufficient NDE (non-destructive evaluation) per Appendix 7 is performed.

Casting purchasers need to understand that in most ASTM specifications these additional NDE requirements are not mandatory. They are simply listed as supplementary requirements at the end of the product specifications and are only invoked if included in the purchase order. Specifying additional NDE-like radiography or dye penetrant inspection is one way of helping ensure the quality of valves being purchased. However, a more cost-effective way is to deal with valve suppliers who already control the quality of the products they produce and have a long and successful track record. In either case, the decision to go with cast or forged valves depends on several factors, and cost is usually the determining one.


THOMAS SPENCE is director of materials engineering for Flowserve Corp. (www.flowserve.com), Dayton, OH. Reach him at This email address is being protected from spambots. You need JavaScript enabled to view it..

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