July/August 2011
Your Letters

Follow-up and maintenance required

Thanks for this great article, John Renick ("Improving decision-making for equipment assets," May/June 2011 InTech). As a Virginia Tech Engineering alumni who participated in related research projects and an active participant in many strategy management and industrial dashboarding projects (using www.ClearPointStrategy.com vs. AMS Suite), I could not agree more with what you illustrated here. Investments and new initiatives should guide the way ahead, yet without sufficient follow-up and maintenance, they fail all-too-often (as your Wall Street Journal quote perfectly illustrated). Software and advanced analytics will not result in success alone, but coupled with strong leaders and an informed and dedicated team, shared data and analysis definitely contribute to increased up-time, effective initiative implementation, and ultimately mission-based success.

Thanks again for this concise article!

Jeremy

 

Control valves online query

I have a technical query on a 2003 article that I found on InTech's website ("Seat leakage standard revised" by Tony George).

1. Can we apply and specify Standard FCI 70-2-2006, Class-VI for a full port 54-inch butterfly valve as "Seat Leakage Classification"? The Table-2 provided on Page 3 of Standard FCI 70-2-2006 limit the Class-VI to the "Nominal Seat Diameter" up to 400 mm or 16- inch-this is my understanding.

2. The note below the Table-2 with two asterisk (**) states that "if the valve seat diameter differs by more than 2 mm (0.08 inch) from one of the value listed (Table-2), the leakage rate may be obtained by interpolation assuming that the leakage rate varies as the square of the seat diameter" and hence some VENDOR of 'Control Valve' are manipulating that the FCI 70-2-2006 Class-VI 'Seat Leakage Classification' can be applied for the leakage rate of respective valve, having 'Nominal Seat Diameter' more than 400 mm or 16-inch.

I urgently need your expert opinion that that the Standard FCI 70-2-2006 Class -VI can be applied for the Control Valves, having Nominal Seat Diameter more than 400 mm or 16-inch.

Abid

Response:

A question was raised by an InTech reader with respect to applying the Class VI leakage criterion of ANSI/FCI 70-2-2006 to a 54-inch size butterfly valve as this size range is beyond the scope covered by the standard. Also, applying a stringent leakage control valve leakage class to a valve with a linear sealing length of approximately 169 inches could present a significant manufacturing challenge.

In any relationship between a purchaser and a manufacturer, the criteria to which the product is constructed or tested can be defined by the specifications incorporated into the contract.

In formulating such specifications, it is of value to attempt to evaluate the basis for the allowable values in the standard under considerations if a realistic assessment is to be made for extrapolating those values and limits to a significantly larger size.

Class VI leakage in the ANSI/FCI standard is based on an allowable leakage in ml per minute for nominal seat diameters varying from 25 mm (1 inch) to 400 mm (16 inches).  Although there are some anomalies at the smaller sizes, once the size equals or exceeds about 4 inches, the permissible leak rate is proportional to the square of the diameter. This would indicate the allowable leakage is a function of Cv, the flow coefficient, as the flow coefficient is a function of the square of the diameter.  

This can be explored in more depth, since the standard is believed to have been originally written for globe style control valves with a Cd value, the characteristic Cv of a 1 inch size valve, of about 10.6.

On this basis, the Cv of a 4-inch size globe style valve would be approximately 170. If we apply a test pressure of 50 psi to this valve, the calculated full flow capacity would be 1.7 x 107 ml per minute. The standard permits an allowable Class VI leakage rate for this valve size of 1.7 ml/minute. By analogy with Class IV leakage criterion, which is based on 0.01 percent of full-rated flow, the allowable leakage Class VI leakage translates to 0.000001 percent of full-rated valve capacity. This relationship holds through the size ranges up to 400 mm (16 inches).

Butterfly valves exhibit significantly higher Cd values than a linear globe style control valve. If we assume a Cd of 25 for a butterfly valve, then a 54-inch size butterfly valve would have a Cv of approximately 73,000. The full flow rate for a pressure differential of 50 psi would be 73,200,000,000 ml per minute.

For a permissible leak rate of 0.000001 percent of full capacity, leak rate = 732 ml per minute.

However, as stated in the original InTech article (www.isa.org/link/Seat_Oct03), the permissible leak rates when based on a percentage of the full flow Cv values (with Cv being proportional to the square of the nominal pipe or orifice diameter) become progressively more permissive as the size increases compared to Class V leakage, which is based directly on the diameter. In addition, no limitations are placed in the standard regarding size for Class V leakage.

Class V leakage = 4.7 ml per minute per inch of diameter = 4.7 x 54 = 254 ml per minute.

In constructing a specification for air leakage testing for this valve, there are three possible options for consideration based on the required level of leak tightness assuming resilient seating:

AIR TESTING

Class VI leakage

Direct conversion for size: allowable leakage = 28.4 x (54/16)2 = 323 ml per minute

Converted for size and an assumed Cv of 73,000: allowable leakage = 732 ml per minute

Class V leakage

Permissible leakage = 4.7 x 54 ml per minute = 254 ml per minute

In view of the fact that Class V was believed to be originally intended for tight shut off for hard seat valves to be used in the nuclear industry and the challenges of maintaining the tolerances, flatness, concentricity, and alignment of the mating surfaces with the periphery of a 54-inch size valve, the larger allowable leakage options may be more appropriate.

Tony George