Your safety valve should not be opening unless you are above of your design setpoint. You should be able to adjust it within a range. It may be operating correctly and you need a pressure gage to monitor what is happening. It may need more control of the grate and combustion air. The reason some boilers have two or three safety valves is one might protect a super heater and the other might be on the drum. You always want steam flow through the superheater so its set to go first then the drum goes. So take some time with this problem it needs to be correct and you need to watch this problem.
Boiler for putting around in small dinghy.
- Thread starter rolphill
- Start date
Help Support Home Model Engine Machinist Forum:
HMEL: img-1185 shows the pressure gauge (in shadow) at just over 5 Bar. - While the Safety relief valve is blowing-off.
As (I understand) Rolphil was doing a steam test to ensure "full fire" - "with full blower" can be managed by the Safety Relief Valve, I think he was demonstrating it is about right, but he is planning more tweaks and should confirm the Steam test after each tweak, to be sure it is still OK.
To manage the water "priming" that he has when the safety relief valve "pops", I have seen some boilers use an additional smaller or "gentle-Pop"-type of valve set slightly below the main Safety relief valve pressure. This controls the pressure without priming, but cannot contain full fire with full blower, so the designated (larger?) "Safety relief valve" ensure it is SAFE. (but then doesn't prime). - Particularly on model locos where there is a small ullage and priming would occur if only a single large "rapid-pop" type Safety relief valve were fitted.
Many ways to kill a pig. (and we are trying to AVOID killing the Boiler-operator..).
K2
As (I understand) Rolphil was doing a steam test to ensure "full fire" - "with full blower" can be managed by the Safety Relief Valve, I think he was demonstrating it is about right, but he is planning more tweaks and should confirm the Steam test after each tweak, to be sure it is still OK.
To manage the water "priming" that he has when the safety relief valve "pops", I have seen some boilers use an additional smaller or "gentle-Pop"-type of valve set slightly below the main Safety relief valve pressure. This controls the pressure without priming, but cannot contain full fire with full blower, so the designated (larger?) "Safety relief valve" ensure it is SAFE. (but then doesn't prime). - Particularly on model locos where there is a small ullage and priming would occur if only a single large "rapid-pop" type Safety relief valve were fitted.
Many ways to kill a pig. (and we are trying to AVOID killing the Boiler-operator..).
K2
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Rolphil,
I suggest you mark a clear RED_LINE at the designated Normal (max) Working Pressure. Then it is CLEAR for any operator to read that it is not exceeded when the Safety Valve is relieving (or not).
You could also do yourself a favour and add a simple tube around the valve so it deflects all the steam and vapour upwards, away from eyes and others?
Keep posting,
K2
I suggest you mark a clear RED_LINE at the designated Normal (max) Working Pressure. Then it is CLEAR for any operator to read that it is not exceeded when the Safety Valve is relieving (or not).
You could also do yourself a favour and add a simple tube around the valve so it deflects all the steam and vapour upwards, away from eyes and others?
Keep posting,
K2
rolphill
Active Member
Yep. I made a non-pop relief valve yesterday. The poppet is made out of teflon as I didn't have any ball bearings laying around. It took a few different attempts to get a poppet shape that worked well, although it still weeps a bit sometimes. My hunch is that steam temps will soften the teflon and allow a better seal. This is installed in addition to the pop valve, which seems to open around 90psi, and close around 65-70 psi. This valve hasn't been used much, and I've heard these kinds of valves tend to do that at first and then settle down to their proper rating. In any case, I still plan on doing another hydro test to 200psi as soon as I get a gauge that goes that high, and replacing it with a 100psi pop valve.
The significant amount of water carryover was certainly not helped by the water quality. I was using my well water from the hose faucet for the initial tests, pre-softened. My well water has a TON of iron in it. You could probably evaporate a small swimming pool of it and have enough iron to make a sword. My water softener works very hard. You can see if you look closely at the water gauge, it's all fouled up with iron already. Luckily these commercial water gauge fittings are super easy to pull the glass out of. Very well designed. We have a deionizer at work for the CNCs that I'll probably borrow some water from in the future though. I also need to get some TSP or whatever at some point.
rolphill
Active Member
Another quick steam up this morning. This coal is very nice, it lights almost instantly when placed on a hot fire. Carryover was significantly reduced with deionized water, but all the impurities still have to work their way through the blowdown and such. I might have to upgrade from fiberglass insulation to mineral wool, as my aluminum outer shell is browning in a nice ring just above the top of the firebox. I'll probably have to make the insulation layer thicker in this area. Oh and the plastic completely boiled out of the firedoor hinge. I adjusted the fire door "spring" and it stays closed without any gaps anyways now. The bolt I used for a handle got real hot though.
I haven't worked on the blower yet, but I tried opening the blower valve all the way, and I lost steam pressure as it vents faster than it can draft at full open. It also likes to gurgle and pulse every time I adjust it.
This boiler is about the upper limit for hand pumping, and I'm glad my hand pump has such a long lever. I'll also be glad when I hook up the engine pump. It's too bad injectors are rather expensive.
My homemade relief valve is... subpar, which wasn't particularly surprising. It really didn't want to close without tapping, and liked to leak. The main pop valve I have has settled down now, and started popping at 90psi and closing at 80psi.
I grabbed a 1/8npt pop valve I had, and did some surgery on it. Normally these are staked so you can't mess with them, so I milled out the staking. This valve is rated for 125psi, which is an odd number. I think I used it on my very first monotube. The commercial valves have a small raised ring for the seat, and a captive silicone disk for the poppet. They also have some extra flange to increase the "pop" aspect. I'm going to take this smaller pop valve and machine off the extra flange, and re-adjust it for 80psi or so. Then I'll test it on air. I also want to see how it behaves if I put a needle valve in front of it. If the needle valve works, I'll install an orifice in the bottom of the relief valve.
I haven't worked on the blower yet, but I tried opening the blower valve all the way, and I lost steam pressure as it vents faster than it can draft at full open. It also likes to gurgle and pulse every time I adjust it.
This boiler is about the upper limit for hand pumping, and I'm glad my hand pump has such a long lever. I'll also be glad when I hook up the engine pump. It's too bad injectors are rather expensive.
My homemade relief valve is... subpar, which wasn't particularly surprising. It really didn't want to close without tapping, and liked to leak. The main pop valve I have has settled down now, and started popping at 90psi and closing at 80psi.
I grabbed a 1/8npt pop valve I had, and did some surgery on it. Normally these are staked so you can't mess with them, so I milled out the staking. This valve is rated for 125psi, which is an odd number. I think I used it on my very first monotube. The commercial valves have a small raised ring for the seat, and a captive silicone disk for the poppet. They also have some extra flange to increase the "pop" aspect. I'm going to take this smaller pop valve and machine off the extra flange, and re-adjust it for 80psi or so. Then I'll test it on air. I also want to see how it behaves if I put a needle valve in front of it. If the needle valve works, I'll install an orifice in the bottom of the relief valve.
Hi Rolphil,
I like your optimism with the way you modify bits "ad hoc"...
I prefer to take a more scientific approach - if I can find the information and make it work.
re:
"I haven't worked on the blower yet, but I tried opening the blower valve all the way, and I lost steam pressure as it vents faster than it can draft at full open. It also likes to gurgle and pulse every time I adjust it." - Well, all I can suggest is initially yo halve the cross-sectional area of the blower nozzle, and re-try. But also check the effect of the blower when you only have about 30psi, when first developing steam pressure. (I assume the engine is pretty useless below that?) - or at minimum engine functioning pressure. The intention of a good blower is:
Depending on how the testing goes, you can do a nozzle "C" at either half way between nozzle "A" and "B", or half CSA of nozzle "B". - Then try again. (lots of fun!).
I hope this gives some direction to your "development method".
You could of course resort to calculations, but I feel you are not that sort of guy?
But regards your Safety Valve mods.
I have "played-about" and used some graphical indications to tweak some crude safety valves to be more "pop"-like.
A simple valve is like the "Big switch-back" shapes, the better "pop-type" (called "soft-pop" by the designer) is the flatter, smoother shape. Owing to the bits I am modifying, I can't always get close to the ideal ratios of cross-sectional area at the critical points through the valve, but the closer I can get the better the hysteresis (difference between opening and closing pressure... less than 4% is very good, more than 10% is harder to live with). The diameter you have reduced increases the cross-sectional area for steam to pass at the second graph point, so making the hysteresis worse, but action "softer" - so less likely to prime... But it may be very poor at closing. What happens is that the dynamic pressure on the valve is different to the static pressure. It needs static pressure in the boiler to open the valve, but then the steam passing through (plus condensate as it expands, plus carry-over priming water!) causes a dynamic pressure that is higher, thus keeping the valve open as the pressure drops lower. You are right in thinking that valve outside diameter is critical, but you may have gone too far....? Just a few thou at a time may have been better...?
I look forward to hearing of your further experiences and testing of this boiler.
Incidentally, the 125psi may be for an air compressor that works up to 8Bar...? = 5 psi opening over 120psi? Safety valves on air receivers do not experience much dynamic pressure for "water" in the exiting air... so generally are simpler (no "pop" action). And as they have electric motors and pressure cut-offs fitted, they usually stop pumping so the safety valve never operates...
Please feel free to ask when you need to. I am more of a "has been under pressure" ex-spurt than a "knowledgeable" Expert... but willing to throw in some ideas.
K2
I like your optimism with the way you modify bits "ad hoc"...
I prefer to take a more scientific approach - if I can find the information and make it work.
re:
"I haven't worked on the blower yet, but I tried opening the blower valve all the way, and I lost steam pressure as it vents faster than it can draft at full open. It also likes to gurgle and pulse every time I adjust it." - Well, all I can suggest is initially yo halve the cross-sectional area of the blower nozzle, and re-try. But also check the effect of the blower when you only have about 30psi, when first developing steam pressure. (I assume the engine is pretty useless below that?) - or at minimum engine functioning pressure. The intention of a good blower is:
- The blower will recover a freshly watered boiler and freshly fuelled fire when the old fire has burned too far down and pressure has dropped to the minimum where the engine is effective, so the regulator has been closed... On a loco, the fireman would be told by the driver the minimum pressure needed for each rise of the track on the route, so the fireman could learn when to get get the fire fuelled, and boiler watered, and still have enough steam so as not to stall on the hill. It may also be the lowest pressure where the injector functions adequately.
- The blower should not be too big, so forcing the fire to generate too much steam for the safety at fully open, when the regulator is fully closed.
Depending on how the testing goes, you can do a nozzle "C" at either half way between nozzle "A" and "B", or half CSA of nozzle "B". - Then try again. (lots of fun!).
I hope this gives some direction to your "development method".
You could of course resort to calculations, but I feel you are not that sort of guy?
But regards your Safety Valve mods.
I have "played-about" and used some graphical indications to tweak some crude safety valves to be more "pop"-like.
A simple valve is like the "Big switch-back" shapes, the better "pop-type" (called "soft-pop" by the designer) is the flatter, smoother shape. Owing to the bits I am modifying, I can't always get close to the ideal ratios of cross-sectional area at the critical points through the valve, but the closer I can get the better the hysteresis (difference between opening and closing pressure... less than 4% is very good, more than 10% is harder to live with). The diameter you have reduced increases the cross-sectional area for steam to pass at the second graph point, so making the hysteresis worse, but action "softer" - so less likely to prime... But it may be very poor at closing. What happens is that the dynamic pressure on the valve is different to the static pressure. It needs static pressure in the boiler to open the valve, but then the steam passing through (plus condensate as it expands, plus carry-over priming water!) causes a dynamic pressure that is higher, thus keeping the valve open as the pressure drops lower. You are right in thinking that valve outside diameter is critical, but you may have gone too far....? Just a few thou at a time may have been better...?
I look forward to hearing of your further experiences and testing of this boiler.
Incidentally, the 125psi may be for an air compressor that works up to 8Bar...? = 5 psi opening over 120psi? Safety valves on air receivers do not experience much dynamic pressure for "water" in the exiting air... so generally are simpler (no "pop" action). And as they have electric motors and pressure cut-offs fitted, they usually stop pumping so the safety valve never operates...
Please feel free to ask when you need to. I am more of a "has been under pressure" ex-spurt than a "knowledgeable" Expert... but willing to throw in some ideas.
K2
Last edited:
I can see the discoloration of the water in the gauge...
I agree, the dirty water will be a problem by priming! - and also creating "Mud" in the bottom of the boiler, and low points in tubes containing water. Frequent cleaning and blow-down needed!
Best idea is to fill with pre-filtered water. (e.g. Rain water that has had time to clear by settling? - melted snow?).
K2
I agree, the dirty water will be a problem by priming! - and also creating "Mud" in the bottom of the boiler, and low points in tubes containing water. Frequent cleaning and blow-down needed!
Best idea is to fill with pre-filtered water. (e.g. Rain water that has had time to clear by settling? - melted snow?).
K2
