Minimum PSI for Practical, Peek Poppet/Valve Seat Usage

[SpiralGroove]

Hey Guys,
It has been well documented, using Peek for Poppet/Valve Seat material can increase PCP valve efficiency.

However, What is the Practical Minimum PSI "Rule of Thumb" with a Peek set-up?.

I mainly shoot regulated guns with set-points ranging from 110 to 125 bar.  Does it make sense from an efficiency/longevity standpoint to use Peek over Delrin?

Or am I asking for sealing issues, unless I achieve a precision Poppet/Seat marriage?

Sorry, for the rambling series of questions .....   

[nervoustrigger]

Kirk, I'm quite happy to use it down at 800 - 1000psi in some cases.
 
If you can get the surface quality good enough, a PEEK poppet will seal straight away at low pressure so there really is no practical lower limit.  Granted the advantage of being easy to knock open isn't as beneficial at low pressures, and may therefore not outweigh the difficulty of getting a good seal.  I've certainly had experiences where I've had a very slow leak that required me to tear it down to have another go at surfacing it.
 
It's like anything, you get better at it with practice.  Nowadays I usually have success on the first try by dressing the sealing face with a simple jig.  It's nothing more than a block of wood with a hole drilled in it that matches the stem diameter.  Then I ready a couple of pieces of sandpaper by punching some holes that the stem will pass through.  Then I insert the stem through the sandpaper and through the wood block and grab the end of the stem with a drill and spin it.  220 grit first until the machining marks are gone.  Then 400 grit to smooth it.  Then lastly I lap it a bit in the valve with J-B bore compound.
 
I think the difficulties people sometimes have is partly due to the valve itself.  The surface quality of many valve seats isn't sufficiently good to seal properly against such an incompressible plastic.  That's where the lapping step takes on particular importance.  If I can see it needs it, I'll start with a slurry of J-B and shavings from a stick of #2 Porter Cable polishing compound.  That cuts at about 2x the rate of J-B alone.  Then I clean it off and do a run with J-B by itself to finish up.

[Motorhead]

Agree .... Have had PEEK poppets seal up immediately filling an empty gun with some weeping until 800-1000 PSI is present.

Comes down to TYPE / SHAPE of the seat and mated poppet first and foremost, then comes the concentric and square to one another. Followed by how wide the seat / sealing margin is being in contact with the PEEK
PeeK has such low yield to compression it simply won't shape shift like acetal or pet-t will.  Think of the fit required similar to a 4-stroke internal combustion engines HEAD VALVE ( This is metal on metal BTW )

[SpiralGroove]

Kirk, I'm quite happy to use it down at 800 - 1000psi in some cases.

Nowadays I usually have success on the first try by dressing the sealing face with a simple jig.  It's nothing more than a block of wood with a hole drilled in it that matches the stem diameter.  Then I ready a couple of pieces of sandpaper by punching some holes that the stem will pass through.  Then I insert the stem through the sandpaper and through the wood block and grab the end of the stem with a drill and spin it.  220 grit first until the machining marks are gone.  Then 400 grit to smooth it.  Then lastly I lap it a bit in the valve with J-B bore compound.

Thanks Jason,
Sounds like your advocating a flat, poppet facing []---- mating with the valve seat.
This, as you know, is different vs. a chamfered poppet descending to valve seat - right? 
 

[JuryRigger]

Agree .... Have had PEEK poppets seal up immediately filling an empty gun with some weeping until 800-1000 PSI is present.

Comes down to TYPE / SHAPE of the seat and mated poppet first and foremost, then comes the concentric and square to one another. Followed by how wide the seat / sealing margin is being in contact with the PEEK
PeeK has such low yield to compression it simply won't shape shift like acetal or pet-t will.  Think of the fit required similar to a 4-stroke internal combustion engines HEAD VALVE ( This is metal on metal BTW )

A quick question on that note, as I think it applies to the topic- what would be an average sealing margin for most small-bore valves? A .020 rim? A .030 rim? A general idea, if you would.
Thank you in advance,
Jesse

[mackeral5]

If I had a choice, my valve seat would be tapered/funnel shaped, with the peek poppet base almost a 90 degree corner, barely blunted,  which seals against the valve seat.  OD of the poppet would be just large enough to not fall/wedge into the valve throat.  This is how my P15 and Bullddog(s) are setup and they are very easy to open and flow very well. 

Using this arrangement and with everything true/square and slick sealing surfaces I've run reg settings as low as 90bar. 

But I do fill from a tank, so I've never dealt with the pump from empty challenge....

[nervoustrigger]

Right, if it’s a tapered seat you’ll be relying a bit more on how well you do with the initial machining step, and do the surface dressing by way of lapping it in the valve.  However if you have some machining marks that need to be removed before that step, go ahead and hold sandpaper against it before removing it from the lathe.

[nervoustrigger]

The smallest sealing margin I’ve used is about 0.015” but that was only at about 1500psi regulated.  It’s a 0.260” poppet over a 0.230” throat.  It showed no evidence of extruding when I had it apart after maybe 500 cycles.  I suspect it would handle a narrower margin but I’m not confident enough in my lathe fu to push it, and I’m not inclined to think the benefit in flow rate would justify the trouble.
 
To put some numbers to it, the compressive strength of PEEK is listed at around 15,000psi.  In the example above, the force acting on the poppet is about 80lbs (1500psi acting on a 0.260” dia).  The annular ring carrying this load (rim of the poppet) has an area of about 0.0115 sq in.  So the material is seeing a loading of about 7000psi.  That’s only about half of the material’s rating.  Or to say it differently, I could probably get away with 2x the pressure and use this valve in a 3000psi application.
 
The takeaway here should be that we can’t think of it only in terms of sealing margin and operating pressure, but also the size of the throat and poppet since they dictate the applied force and the area of material responsible for supporting the load.

[Motorhead]

Unfortunately the poppet is not sitting there in a static state ... but is dynamic VIOLENTLY impacting the seat upon closing.

This will yield Much much higher compressive forces onto / into the PEEK material.

[nervoustrigger]

Hmm, that thought crossed my mind but I kinda dismissed it.  A valve lifts only a short distance (1/16” to 1/8”) which means it doesn’t have much of a runway to accelerate back toward the seat, so it seems like its velocity will be correspondingly low. It also doesn’t weigh much so it’s not carrying much energy or momentum.  That makes me think it’s not a very big contributor.

Quick math for 1/8in lift and the valve open for 1ms, it will be closing at something substantially less than 10fps.

[Motorhead]

Bob S could likely crunch those numbers.

AKA: Valve recession is very real and comes from IMPACT and very little from sitting there closed in a static state pressure holding it closed.

Having worked quite extensively with PCP valve systems & the Damage noted with assisted closure design such as "Cobra" air springs and Balanced valves in the SS1 SS2 type designs with throat pressure to chamber air springs, what works in a conventional PCP valve will more times than not quickly fail when subjected to this beating !!

[nervoustrigger]

Okay thanks for the words of warning, Scott.  I'll try searching the topic and read up on it.

[Motorhead]

Not sure what your going to find in reading .... other than posts from years past showing poppet recession, pounding into throat that BOB did quite a few years ago.
Most folks don't do the R&D and push design limits and say much about it.  Poppet fails they replace it having little care or clue in the Why For of the failure.

need some help just PM me.

Scott

[mackeral5]

I would love to see some examples of peek poppet failures.  So far I have only been able to damage them with foreign debris, or the one time a big bore sucked the return spring out into the throat and trashed the peek poppet and seat.  I can't imagine what it would take to extrude a peek poppet. 

It is a whole different ball game for the DIY vs the Pro.....the DIY guy only has to make it work once.  When things fail, usually it is just an inconvenience.  After getting over the disappointment I typically enjoy the failures, in most cases there is quite a bit learned. 

For the Pro, it is money lost, reputation damaged, potential future business lost.  Customers typically don't pay for R&D nor mistakes.....

[nervoustrigger]

Scott, yeah I was just thinking the topic of impact trauma to the poppet had been discussed before so figured I'd research it a bit before poking Bob for comment.  I found some discussion in which Lloyd stated:

...can we look at the closing half of the cycle because that seems easiest  to analyze. The main component of closing force on the valve, as has been pretty thoroughly discussed and agreed upon, is the pressure times the valve stem area.  So with a .156 stem dia, and 2000 psi, the closing force is 38 pounds.  If the valve weighs .08 ounce, or .00017 slugs mass, and accelerates from zero at the full open position for 0.10" to the seat, the velocity of the valve when it hits the seat is only 43 ft/sec, which is 29mph.  So even though the closing acceleration was over 6,000g's, the velocity at impact was relatively slow.....I guess.  Those plastic valve heads hold up forever, so the impact on the seat can't be too bad.

He hedges a little bit there at the end with the "I guess" so I think I'll ask Bob to chime in.

You may well be right, I just want to understand the mechanism behind it if so.

[rsterne]

If you want a PEEK (or any) poppet to seal up at the lowest possible pressure, make the head of the poppet concave, coming to a knife edge at the OD.... and run it against a flat seat.... It will deform until the force when closed divided by the seat margin area equals the compressive strength of the poppet material....

I think Lloyd's calculation of the velocity of the valve stem when reaching the seat neglects the mass of the hammer, which the poppet must accelerate rearwards while closing.... IMO the velocity of the poppet when it hits the seat is low, and hence the force required to stop it (the hammer carries on) is small compared to the static load.... The exception may be, as Scott notes, in valves that "blow shut" because the closing force is much greater than normal.... A Cothran valve with the "shuttle rod" removed from the stem is an example.... The very soft poppet can destroy itself from being blown shut by forces it was never intended to see.... 

Bob