Cv vs. Cd, and Kdr are they related and how does mfg not specify on a certified valve the Cd/Kdr?

[adammil1]

I have spent some time reading through Leser's manual primarily the Chapter 7 on sizing of relief valves. Between this and other discussions on this website it would appear to be clear that the mfg needs to give me the Coefficient of Discharge or Kdr (Certified Derated Coefficient of Discharge) as the ISO-4126 calls it. So reading through the document I am pretty certain I can calculate the minimum area required by both ASME, ISO4126 and the AD Merkblatt 2000 so long as I can get the Kdr/Cd which I believe are the same things.

Now I came across a Hoke R6000 relief valve which for my application is lower in cost by a factor of 10 than most other products. The valve meets the PED (Here's the Hoke Cert from TUV Rheinland

http://www.hoke.com/Certifications-Licenses/D of C R6000 Relief Valve.pdf)

but not ASME which since this is for use in Germany doesn't seem to be an issue.

So I went to Hoke's website and started looking around and all they give is the Cv on the valve, which I believe is not applicable here from everything I have read. I called up Hoke and they tell me to go to their flow calculator on

http://www.goreg.com

and put in the Cv, pressures, temp, and out pops the max flow of the theoretical back pressure regulator which they say is equivalent to the relief valve.

From everything I understand this is not how it is done and I should be using the proper calculations from the ISO-4126 or Merkblatt. So why would they publish the Cv if all the designer cares of is the Cd/Kdr?

When it comes to getting the PED cert above is the manufacturer required to publish the Cd's like they are with the ASME? In the case with an ASME valve I have the Redbook on my computer and can look up the Cd, I thought they need to register the Kdr with the PED somewhere too is this the case? If I can prove to the code that the Hoke valve is suitable for the flow I would like to use it as in our quantites there is a very good cost savings, on the other hand I just can't understand why a manufacturer would make and certify a valve and not provide the required information for an engineer to specify this.

Can anyone shed some light here? Can I convert between Cv and Kdr? Or is it just time to dump Hoke and buy from a more reputable PRV manufacturer like Leser or someone?

Thanks,

Adam

 

[don1980]

You asked some good questions. I've also searched to find a European equivalent of the Redbook, and haven't found one. As far as I know, there's no such thing, and one has to get the certified valve coefficients (Kdr) from the valve supplier.

Hoke makes small (tubing size) valves, so I assume you're talking about a very small relief flow. For US applications, you can also find a wide range of these small inexpensive valves. They generally suitable for small scale equipment (lab & pilot-plant). They're generally not used in process plants because of their small size, threaded connections, and comparitively weak mechanical design. In plants we generally want something more beefy, and that's why these industrial valves cost more. Without knowing more about your particular application, I can't say whether or not these small cheap Hoke valves are suitable.

As for your question about the lack of a Kdr, small relief valves like Hoke are commonly sized using a capacity chart or simplified equation. In this case, it appears they're providing a Cv so you can do a simple flow calculation using that Cv equation, rather than using coventional relief valve sizing equations. The fact that the vendor couldn't provide a Kdr just underscores the point that these valves are normally not installed on industrial scale equipment. If this type valve is indeed suitable for your service (small scale, non-process plant application) then I think it's OK to calculate the capacity using the Cv equation. These small valves are proportional modulating type valves - they're fundamentally different from pop-acting PSVs which are sensitive to inlet pressure loss. They're outside the scope of the equations we use for industrial PRVs.

 

[adammil1]

Hi Don,

Thanks for the reply, we are currently scaling up our current product line and starting to become more of an industrial process plant as opposed to our current equipment which is more on the lab/pilot plant scale in size. This will be about 10times bigger than we have produced in the past. As such proper sizing has become more important than ever before where a lot of our lines have quite a bit of extra capacity on the legacy equipment.

I don't want to post too many details here as it is proprietary and under development however for the particular relief valve I will have 3 different places it is needed and the flow will be 133, 266, and 643scfm of hydrogen at 500psi and up to 175 or so degF. As such my calculations are showing me depending on the Kd we are almost right up against the capacity of the Hoke valve, but that it should likely be in the range for the lower flow applications. I would wonder if the Europeans even bother to maintain some form of a Redbook somewhere? On the other hand Kdr still apparently stands for "Certified Derated Coefficient of Discharge" which would lead me to think that somewhere to get that PED cert it should be written down.

Thus leads into the bigger question of what needs to be followed for use in Europe regardless of what valve I actually select? I went out and purchased "The Safety Relief Valve Handbook" which I found on Google books and it has provided one of the best explanations I have seen so far on the PED vs. ISO 4126 etc... One of the interesting things I found in reading the book is that the PED is the law in Europe where as the ISO/EN 4126 appears to be recommendations on how to follow the law. At some point it may make sense to go out and buy the actual PED code and read it to be sure but I am starting to wonder if the PED merely requires properly sized PRV's and than leaves it up to the engineer to follow good recommended practices? Still missing from all of my understanding is where the AD 2000 Merkblatt A2 enters the picture? It would appear as though the PED was supposed to normalize and take precedent over everything in Europe but clearly the AD 2000 Merkblatt still gets maintained based on their website.

http://www.ad-2000-online.de/.

I would really like to come up with a standard company wide procedure to produce the documentation needed to satisfy the European notified body in this case TUV Rhineland that our product is acceptable.

Anyone have any ideas here?