Valve Stem Sizing 3

[Sandro_S]

Hi,

I am brand new to this forum and this is my first post so please bear with me!

I work for a company that manufactures gate valves. These are not exactly your typical gate valves but for my purposes I don't think it will change anything. I have been tasked with creating a spreadsheet to input all of our known information and have it spit out the diameter of stem required to operate the valve. How the valve works is like this:
- An ACME threaded stainless steel round bar is used for the stem. The threaded portion is threaded into a brass drive nut of an actuator above the gate valve.
- The bottom portion of the stem has a clip welded on to it. The clip has two bolt holes in it as well as the sliding door of the valve.
- The stem is bolted to the slide.
- As the actuator turns the brass nut, the slide begins to rise.

I hope this has created a clear visual. Up until now we have been sizing the stem with an "over size everything" approach which has worked so far and been signed off by engineers but we are now looking for the more economical approach. I can calculate the thrust required to operate the gate as per ANSI/AWWA C561-14 but that is as far as my knowledge will take me.

If anybody could help me get started on this that would be immensely appreciated!!

Thanks
Sandro

 

[MikeHalloran]

The stem represents what, a few percent of the valve's mass?
... and because it's not a coppermetal or exotic alloy, a still smaller proportion of the material cost?
Who decided that smaller stems might be a good way to save money?

Suppose you start reducing stem size; how will you know when you've gone too far?
Will you rely on your customers to tell you?
What if they just buy from your competitor?

I propose that your spreadsheet should include information including cost and processing time for every process step for every part. Then you can begin to identify the steps you can remove or streamline or otherwise improve.

In fact, the process of gathering data to fill in such a spreadsheet will probably cause you to stumble over multiple opportunities for cost savings.




Mike Halloran
Pembroke Pines, FL, USA

 

[Sandro_S]

Mike,

Thank you for posting. Your assumptions are correct in some cases however you'd be surprised how often the stem weight meets or exceeds the valve weight. The machining costs also add quite a deal to the associated stem costs. Also, economics is not the only reason. There is also a chance that we might under size a stem which is far more problematic than over spending. I really just need to figure out how to convert my thrust and torque calculations into the right stem diameter for the job. We do not use an engineer on every job because many jobs have very similar applications.

I am assuming that the determining factors will be the yeild strength of the round bar, or the pull out strength of the ACME threads on the round bar or brass nut. I just don't know how to calculate what I need.

Thanks
Sandro

 

[bcd]

If you limit your thinking to stem torque & thrust required to operate the valve, you will get yourself into trouble. The person who proves if your stem is strong enough is the guy (or girl)turning the hand wheel at the end of the stem. Valves get jammed and stuck, and the operator will push the equipment to the limit to get the valve to operate. If any stem failure were to occur from overloading, best it be outside the pressure boundary. MSS has a standard that provides the maximum input capability for operators using hand wheels, levers, etc. I think it is SP-91. The data is based on actual human tests, so it is a good source of information when sizing stems.

 

[Sandro_S]

Hi,

I appreciate any input I can get on this however, I have done all other necessary calculations for thrust and torque based off a maximum rim pull of 40ft/lbs. I only require assistance for putting together a spreadsheet that will spit out the stem diameter required to receive these forces.

 

[ScottyUK]

Sandro,

Your assessment of what you need might be right purely in terms of what you were asked to do, but if you introduce weaknesses to a design - or even reduce its tolerance of abuse as alluded to by bcd - then it is easy to lose a hard-won reputation in a competitive industry.

I see how some of our aging valves behave because my group look after the actuators which drive them. Many require more than the design torque from the actuator because they are old and no longer in factory-fresh condition, and many wear out their drive bushes quite quickly as a result of the excessive thrust they are required to transmit. You may find that following your plan to reduce the stem diameter will also result in a smaller diameter drive thread on the stem which will wear out the drive bush more quickly. The stem thread itself will likely wear more quickly too. Both affect operability and availability: a company might sell an plant a set of cheap valves once but if they cause operational difficulties that company will end up with a bad name and lose repeat business.

 

[Sandro_S]

Hi All,

I must say I truly do appreciate anybody willing to help me with this however I feel like I may have lead everybody down the wrong path. I should never have mentioned any sort of economic value for a spreadsheet to dictate our stem diameter. Ensuring we have at least the minimum diameter required is of far greater importance. I did not stress this heavily because I feel we already over-design the stem as it is. What we are really trying to do is look for some calculations to back our decisions.

After trying to think about our application logically, I feel like the weakest point in the stem would be the minor diameter of the ACME thread. This would mean that the only thing I really must calculate is if the yield strength of stainless steel type 304L or 316L, with the minor diameter of (x), is stronger than my calculated thrust or torque required to operate the valve.

If someone could help me put together a spreadsheet like that I would greatly appreciate it.

Thanks
Sandro

 

[LittleInch]

You need to be careful if the thinnest bit is the ID of the screw thread.

The standard bar tensile force F = A x S is a bit simplistic when you start cutting grooves into it.

The trick is in what factor you apply between the Min tensile strength and your calculated F.

You probably do "over design" them at present, but that over design is built into many valve specs and long established norms and systems.

Your spec noted already seems to include
"Major revisions made to the standard in this edition include:
1. The stem factor calculation, used to convert actuator nut torque to stem thrust, was included."



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Sandro_S]

"LittleInch"

Thanks for the response. I think you are understanding what I am trying to get at. In the end I will most likely be adding an additional safety factor to my calculated stem diameter however I still need to calculate that diameter.

If I could just get someone to tell me a formula or something along those lines that will help me calculate the strength of a stainless steel round bar so I can ensure it will be able to resist the thrust required to lift the gate.

Thanks
Sandro

 

[Sandro_S]

Unless I am missing something, I feel like this should be a fairly easy calculation if you know the numbers to use. Please look at this example and tell me what diameter of stainless steel (304) round bar could handle these forces.

Thrust required to operate gate = 5,726 Lbs
Torque = 93 ft/lbs

We ended up using an ACME threaded Ø1 3/4" stem for the job. After checking the internet for yeild, ultimate, and shear strength calculations, I found this website...

http://www.portlandbolt.com/technical/faqs/calculating-strength/

Using their formulas, this is what I came up with.

Yeild = Cross Sectional Area(used Ø1 1/2" for minor diameter of thread) * Grade Yeild(used 30,000 psi for S.S)
Yeild = 1.767 in2 * 30,000psi
Yeild = 53,000 lbs

Ultimate = Cross Sectional Area(used Ø1 1/2" for minor diameter of thread) * Grade Tensile(used 75,000 psi for S.S)
Ultimate = 132,536 Lbs

Shear = 60% Ultimate Tensile
Shear = 79,522 Lbs

Looking at these numbers it seems to me that we over-designed with an incredible safety factor. This leads me to believe I am doing something wrong. Could someone please point me in the right direction.

Thanks
Sandro

 

[LittleInch]

Axial force in Newtons = tensile strength of the rod (N/mm2) x cross sectional area of bar in mm2

That's for a circular rod / stem in simple tension with no other forces (bending/ torsion etc)

As said the grooves, which is where the force is applied, creates a bending force and shear force on the grooves and it may well be that the limiting force able to be applied is actually a function of the thread and not the bar...



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Sandro_S]

LittleInch,

From your response, It seems like I was doing my math right however as you say these may not be the limiting factors in this whole scenario. If the first thing to fail during gate operation is the threads because of a bending or shear force created by applying force to the threads, then this is far out from my expertise and I have no idea how to calculate this. If you have any idea or could point me in the right direction that would be great!

Thanks
Sandro

 

[MJCronin]

Sandro,

What about other valve manufacturers and their stem diameters ?

Have you purchased your competitor's similar valves and compared design dimensions ?

Are your valve stems the largest amongst your peers or are they all similar ?

I guess that I would want to know if the proposed MBA "savings" from possible smaller valve stems would place you at the smallest stems in your particular industry.

I have purchased many types of valves for the process chemical industry and developed comparison spreadsheets between valve bidders. Having the smallest comparitive diameter stem would count as a negative ....... IMHO

Just my two cents worth ....

MJCronin
Sr. Process Engineer

 

[Sandro_S]

MJCronin,

I agree with your opinion. Yes we are aware of our competitors stem diameters and we are definitely the largest. We do require far more torque than other slide gates to operate to begin with. This may not lead to any savings at all, but at least we will have some mathematical reasons for choosing the stem diameters we use.

 

[LittleInch]

there's plenty of data on acme thread capacity. Things like this

https://www.google.co.uk/url?sa=t&r...d4XsvzdPs_eXZw&bvm=bv.144224172,d.ZGg&cad=rjt

There's even an app for your phone!

https://play.google.com/store/apps/details?id=com.cinotech.acmethread&hl=en_GB

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Sandro_S]

LittleInch

If the weakest link in my stem has something to do with the threads then what you have sent me probably has the answer somewhere in there. I do not have the time to give it my full attention right now but I did take a few minutes to quickly read some sections which resulted in myself getting a massive headache...

The calculations at first glance appear very complex and include variables that I do not know how to acquire like "shear factor". I am a little skeptical about an app but I am willing to try to out however it only runs on android and I have an iPhone...

I will try to carefully read that document over when I have the time although I fear I will not be successful at the end. If you or anybody else already know how to do these calculations and are willing to help me compile a spreadsheet, that would be greatly appreciated!!

Thanks
Sandro

 

[LittleInch]

I just plucked that one because it had some formulae, but there are many others if you just search for ACME thread force calculation or some variant of that.

You need to know the diameter and thread type, but should get the answers fairly easily.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.