LP Gas Tank Safety

[JohnB]

Originally Posted by Jim-Bev-2363

I think the 80% is a feature of the tanks. They have a pop-off valve that bleeds gas once that level is reached so tanks cannot be filled above that amount.

Jim

Just checking as the words you chose could be interpreted a few ways.

As mainah pointed out, a 20# called tank holds 20# of actual liquid LP. The 20# fills the tank to 80% of the cylinder volume so the 20% extra head space allows room for the LP to boil and not accindently spit out in a liquid.

You used the words, they have a "pop-off valve" that bleeds once that level (80%) is reached so the tank cannot be filled above the 80%. I'm picking up on the words "pop off valve", LP tanks do have a saftey relief valve also called a pop off valve however it is gas pressure that opens the saftey relief not level.

A float type valve is used to stop the fill at the 80%.

Did you mean float valve, they call an "OPD" or over protection device in place of a pop off valve?

Here is the OPD. Propane OPD - Overfill Prevention Device Cylinder Valves

That site has a lot of other good reputable info on it.

[Gene]

Originally Posted by Jim-Bev-2363

....
Now that I'm old, fat, bald and retired I just want to get my gas before I pass gas waiting in line.

Too funny

[Jim-Bev-2363]

Originally Posted by Gene

Too funny

But I was serious.

[mainah]

Originally Posted by JohnB

Jim

Just checking as the words you chose could be interpreted a few ways.

As mainah pointed out, a 20# called tank holds 20# of actual liquid LP. The 20# fills the tank to 80% of the cylinder volume so the 20% extra head space allows room for the LP to boil and not accindently spit out in a liquid.

You used the words, they have a "pop-off valve" that bleeds once that level (80%) is reached so the tank cannot be filled above the 80%. I'm picking up on the words "pop off valve", LP tanks do have a saftey relief valve also called a pop off valve however it is gas pressure that opens the saftey relief not level.

A float type valve is used to stop the fill at the 80%.

Did you mean float valve, they call an "OPD" or over protection device in place of a pop off valve?

Here is the OPD. Propane OPD - Overfill Prevention Device Cylinder Valves

That site has a lot of other good reputable info on it.

The tanks have a bleeder valve that has to be opened in order to fill it as the tank fills it allows the liquid to enter the tank because the liquid does not compress and there has to be space for it to fill. About the time the bleeder starts to spit off the vapor the OPD will shut off the the flow through the tank valve at that point you should have 20# of liquid propane in the tank and yes the tanks are larger that just being able to hold 20# of liquid. Every once in awhile we would run across tanks with stuck OPD valves I never was able to understand how they became stuck maybe from storing them upside down or on their side but we would have to slam them down on a piece of 6X6 to jar the valve loose so we could fill them. I can say they really do a good job of shutting the tank off because you could not get then to accept any propane. As far as over pressure I'm not really sure how they deal with that perhaps it lifts the shut off valve and about the only way they would over pressure is being in a fire and I'm not sure I would want to be around to watch unless I had a good pair of binoculars.

[Jim-Bev-2363]

John, I guess I was calling the bleeder valve a pop-off valve since that was to indicate the full level when liquid would come out. It has been 15 years since I filled a tank.

As for the original intent of this thread I do think it is important for us to keep an eye on the condition and age of our tanks. Then whenever repair or replacement is required don't hesitate. The easiest thing to do for 20# tanks is to swap them at a exchange station. Larger tanks will require more specialized repair.

[JohnB]

Originally Posted by mainah

The tanks have a bleeder valve that has to be opened in order to fill it as the tank fills it allows the liquid to enter the tank because the liquid does not compress and there has to be space for it to fill. About the time the bleeder starts to spit off the vapor the OPD will shut off the the flow through the tank valve at that point you should have 20# of liquid propane in the tank and yes the tanks are larger that just being able to hold 20# of liquid.

snip...

As far as over pressure I'm not really sure how they deal with that perhaps it lifts the shut off valve and about the only way they would over pressure is being in a fire and I'm not sure I would want to be around to watch unless I had a good pair of binoculars.

2 good points here to discuss.

The bleeder valve, very good point. I can tell you are one of the trained people who fills tanks by the book. I'm not a trained LP tank filler but have tried to research and understand it. From that research there are several ways to insure a tank is not overfilled. 2 of them are the correct way and should be followed. However we will get to the "should be" in a moment.

A. Fill by weight
B. Fill by the bleeder
C. The OPD shuts off the fill at 80%

You tell us what the regs say or how you were trained however I "thought" refilling stations are suppose to fill by weight and the bleeder. 2 methods to be used to insure there was not a chance of overfill. As overfill can be dangerous if liquid LP shoots out of an appliance. The OPD is there as a saftey back up, not meant to be the primary device to stop filling.

Now to your point about bleeding to allow new LP into the tank. That is a good point of discussion and I feel what your are saying is correct, however it is not what my local tank filling place does by practice.

Here are some thoughts given the situation. If the tank is empty, the pressure in the tank "can" be fairly low in pressure. When the refilling station fills the tank they have a pump, and that pump has a certain head pressure to allow it to pump into the tank. On purpose they limit the head pressure. If the internal tank pressure is higher than ability of the pump to supply, no liquid is going to enter the tank regardless of fill volume. When you bleed, you are creating a lower pressure in the tank, the pump can overcome the internal pressure and liquid goes into the tank. When liquid spits out the bleeder, you know it reached the 80% mark and the scale should read the correct weight at the same time (approx). You shut off the bleeder, shut off the fill nozzle, shut off the tank valve, unscrew the fill nozzle that bleeds a little pressure in the nozzle off and your done. Did I get that right?

Now, those who do not follow that process, like I believe you where trained to, they may not do anything with the bleeder, right wrong or indifferent. They just don't. If the tank pressure is lower than the pump head pressure the tank fills. If the OPD works, the tank stops filling when the float shuts it off. While not the proper procedure, I have seen it done. They may have a scale in the process however the weight may be close but not right where it should be. Since there was no bleeding going on, and the tank starting pressure may be higher, they might hit a point where the pump will no longer pump against the internal tank pressure and no more liquid enters and this might be a pound or so short of 80% full. So the customer gets short changed. Outdoor temperatures affect the internal tank pressure. A hot summer day creates a starting higher internal tank pressure then in the winter. If the pump is sitting there running and running and the weight is not rising, no bleeder open, well it may have reached the pump head pressure stall point. I have seen this too.

I know this may not be what you do, however have you seen it or know of it? Those filling stations, (not LP gas company ones) who have a side line business of filling tanks may not have attendants who understand all the reasons behind the procedure. The main owner of the garden shop etc may, but not their hired help. I have seen this too. The owner uses the bleeder and the hired help does not.

To the saftey relief. This is a separate device. It must have a spring loaded check valve saftey relief separate from any filling device. This I know this from pressure vessel code. A LP tank is classified ASME unfired pressure vessel complete with a U stamp certification. The tank has a MAWP, (Max Allowable Working Pressure) and the saftey relief system needs to open up when it exceeds this value. On a LP cylinder, I "think" this saftey relief in side of the brass valve on top. On the opposite side of the bleeder, there is a small round vent looking device. Behind that vent I believe is the spring loaded saftey relief check. Since in the summer tank pressure can go way up, the saftey has to be in place all the time, not only when filling. And since this saftey can relieve to protect the tank, it is venting gas when it is doing it. Again why they say, do not store LP tanks in side, if the tank vents from over pressure, a fitting leak or a faulty relief valve venting, it can create a saftey situation.

Good discussion.

Thanks

John

[Jim-Bev-2363]

If the tank is not vented and the filling pump stops that means the pump has reached it's limits and less to do with proper full tank limits. Pump controls are internal to the pump, not the tank. In my mind.


Your local filling station not venting is not allowing the tanks to get 80% liquid but is rather compressing the tanks existing gas. Once your tank reaches a pressure, not a volume, the pump stops.

A proper filling station will have a gage on the pump to tell how many gallons is pumped? If weight is so important must a motorhome be weighed for proper filling? I never had mine weighed for LP charges. They used a gage on the pump watching gallons pumped and the liquid coming out the vent to know when the tank was full. LP sold by weight in small tanks is still converted to gallons for paying.

The OPD valve was added as a secondary safety feature, not a full tank indicator. That job is still done by the vent valve. I think the float valve part of the OPD is more to protect against improper positioning of the tank. For instance laying a vertical tank on the side would cause liquid flow and be dangerous. The float would interrupt the flow of liquid in the improper position and as such is a safety devise for position other than for pressure or liquid volume.

The way I was told to sequence operation when filling was to: 1 - connect filling hose to tank. 2- open tank vent. 3- turn pump on. 4- open valve on filling hose. 5- Watch vent for liquid in the vented gas. 6- when liquid comes instantly close fill hose valve. 7- Turn off pump. 8- Close tank vent valve. 9- Mark down how many gallons were pumped. I think that was all of it. Never had to use a scale for weights.

[Jim-Bev-2363]

JohnD, You said "The tank has a MAWP, (Max Allowable Working Pressure) and the saftey relief system needs to open up when it exceeds this value.".

Wouldn't/Shouldn't the relief system open before "it exceeds this value"? I think relieving pressure that is higher/exceeding the MAWP would/could be helping after the explosion has happened.

[JohnB]

Originally Posted by Jim-Bev-2363

JohnD, You said "The tank has a MAWP, (Max Allowable Working Pressure) and the saftey relief system needs to open up when it exceeds this value.".

Wouldn't/Shouldn't the relief system open before "it exceeds this value"? I think relieving pressure that is higher/exceeding the MAWP would/could be helping after the explosion has happened.

Jim,

In the world of modern day pressure vessels they have many ratings and different reasons. The MAWP limit is not the failure limit.

To help explain this, the MAWP (Max Allowable Working Pressure) with key word "working". This is like a WD bar, a 1,000# WD bar means you can use it up to 1,000#. Same for the MAWP. That is the max working rating. So now the user knows where they can safely use the tank up to that rating. And many tanks also have a temperate rating to go with the pressure rating pending the use of the tank.

The saftey relief comes into play to protect the user and the tanks from exceeding the working rating. The relief valve is suppose to be sized to relieve the max item or items that are causing the rise in pressure.

There are at least 2 other ratings higher in pressure above MAWP. One is the hydrotest pressure or test pressure. This rating can be 30 to 50% higher than the MAWP, and uses formulas for temp and pressure of what you are using the tank for to create the test pressure. They use this hydrotest pressure to certify the tank and check for leaks during manufacturing. This higher test pressure will not damage the tank but will check for leaks so the end user does not have any problems during use.

There is then the structural failure limit. This is even above the hydrotest limit. It takes into account the temperatures, materials used, pressure and other attributes. When you meet this limit permanent failure can start.

In the early 1900's and before then when pressure tanks starting rupturing and people getting hurt, ASME created the pressure vessel code and is now law (OSHA) that people who use or buy pressure vessels must have certified tanks.

Thanks

John

[JohnB]

Originally Posted by Jim-Bev-2363

If the tank is not vented and the filling pump stops that means the pump has reached it's limits and less to do with proper full tank limits. Pump controls are internal to the pump, not the tank. In my mind.


Your local filling station not venting is not allowing the tanks to get 80% liquid but is rather compressing the tanks existing gas. Once your tank reaches a pressure, not a volume, the pump stops.

A proper filling station will have a gage on the pump to tell how many gallons is pumped? If weight is so important must a motorhome be weighed for proper filling? I never had mine weighed for LP charges. They used a gage on the pump watching gallons pumped and the liquid coming out the vent to know when the tank was full. LP sold by weight in small tanks is still converted to gallons for paying.

There are at least 2 way to sell LP, by metered gallon or weight of LP.

As you said, small LP tanks can be filled and sold by weight and in our area this is common. A certificated scale is used. The filling attendant knows the tare weight of the cylinder and then fills XX pounds of liquid into the tank. A 20# tank gets 20 lbs of LP, a 30# tank get 30# etc. This works for portable tanks. They have no meter on the pump, they use the certified scale.

Not very good for a motorhome. For the MH or other vehicles, you need to find a place that sells by metered gallon.

To the comment on the pump pressure and the tank, I was only listing that when filling places do not vent you "may" be pressure bound and not fill the tank full. Does not mean it will happen every time, however on hot summer days it can more likely happen then cold winter days. The pump should not be able to exceed the working pressure of the LP tanks so I'm assuming it is lower pressure. I do not know by how much. LP may not compress a lot however it is still a liquid that will not transfer if the tank and pump are at the same pressure.

The OPD valve was added as a secondary safety feature, not a full tank indicator. That job is still done by the vent valve. I think the float valve part of the OPD is more to protect against improper positioning of the tank. For instance laying a vertical tank on the side would cause liquid flow and be dangerous. The float would interrupt the flow of liquid in the improper position and as such is a safety devise for position other than for pressure or liquid volume.

The way I was told to sequence operation when filling was to: 1 - connect filling hose to tank. 2- open tank vent. 3- turn pump on. 4- open valve on filling hose. 5- Watch vent for liquid in the vented gas. 6- when liquid comes instantly close fill hose valve. 7- Turn off pump. 8- Close tank vent valve. 9- Mark down how many gallons were pumped. I think that was all of it. Never had to use a scale for weights.

This site which has a level of creditably to it talks about the OPD only being intended as a saftey to not overfill a tank. If the tanks tips side ways, the float may still be open and let LP out of the tank.
Propane OPD - Overfill Prevention Device Cylinder Valves

What you describe on how you recall filling tanks is what I have learned is correct for when LP is sold by the metered gallon. Pending the area where LP is sold, they also sell it by weight from a certifed scale.

[Jim-Bev-2363]

Thanks John.

[JohnB]

This has been a good post Jim. In our area, finding places that fill by the pound or gallon is job to find them. There are the exchanges however I want to keep my tanks. And then we have one place that fills by flat rate. I only used them twice. The 1st time was to refill my BBQ 20#er for home. The flat rate on this one fit and did not think anything about it. That was until I brought my 5#'er in to get refilled I use on the camper stove. They soaked me the same 20# flat rate. That was the last visit.

The place I use now fills/charges by the pound and it is hit and miss on which attendee opens the bleeder. Soon we will be moving and I have found another place 20 miles up the road that fills by the pound so we will see how that goes.

Curious on mainah's thought on this bleeder part.

Thanks

[mainah]

I can tell you that you cannot fill a tank if the vent is not opened so I'm going to say about a 100 PSI is all the pump is good for nor am I sure the pump does anything more than transfer liquid propane because the pickup is on the bottom of the storage tank so it's at static propane pressure. I am also not sure temp has a lot to do with internal pressure as propane boils at -44F I suppose at some point temp becomes a problem but filling them in the summer poses no difference as to filling them in the winter I fill mine at the camp ground year around. All of the supply tanks I have seen for dispensing propane come equipped with a scale most people however use the meter it’s easier and if it's a fixed tank you can't weigh it.

[Jim-Bev-2363]

JohnB, A suggestion. When tank is filled without opening the bleeder you could request the bleeder valve be opened, as it should be and see how much more you get into the tank. Might not be but a gallon, if that much, but when your Winter camping that gallon could make for some good sleep.

mainah, Outside temperature never made much difference for us. Only the difference in liquid volume which was almost nothing in the tanks used on forklifts. Even our freshly filled tanks did not blow the relief valve when set out in the sun. We filled tanks to the liquid limit and set them in the sunshine all the time. I thought the relief valves would only work if external heat from a fire or other higher temps was applied since no other heat seemed to trip them.


Edited to add: mainah might even experiment while filling a tank. With vent closed, run pump to fill until cannot fill anymore. Then open vent valve to check for liquid. If none then fill till liquid appears. Check how much liquid is added.

[Jim-Bev-2363]

Bits of information found online:

20# LP tank relief valves are set at 250PSI.

One gallon of propane liquid equals 270 gallons of LP vapor.

Pressures of LP at different temperatures: 70 degrees = 96 PSI. 100 degrees = 127 PSI. 110 degrees = 230 PSI.

[JohnB]

Originally Posted by mainah

I can tell you that you cannot fill a tank if the vent is not opened so I'm going to say about a 100 PSI is all the pump is good for nor am I sure the pump does anything more than transfer liquid propane because the pickup is on the bottom of the storage tank so it's at static propane pressure. I am also not sure temp has a lot to do with internal pressure as propane boils at -44F I suppose at some point temp becomes a problem but filling them in the summer poses no difference as to filling them in the winter I fill mine at the camp ground year around.

Thank you. I'm now starting to put the pieces together. Duh... I forgot about the laws of physics going on with LP.... Thank you for reminding me. Before I summarize what I "think" is going on, can you please confirm the bleeder operation as you see it.

Please explain the bleeder operation "during" filling.

Is it just slightly cracked open during the "entire" fill?

Or

Do you only burp it when the scale weight approaches the correct fill weight to check level?

If you do have it open during fill, how far open is the bleeder cracked open? It would be blowing like all heck if open very far.

Before I summarize what I "think" is going on... wanted to check the basics.

One big underlying point I forgot about and you reminded me of. The vaporization temperate/pressure of LP. This changes a lot about what the pump does and does not have to do. For those following along, here is a good explanation of what is going on inside the tank with vaporization of LP.
http://www.propanesafety.com/uploade...r%20Demand.pdf

Thanks

John

[JohnB]

Originally Posted by Jim-Bev-2363

JohnB, A suggestion. When tank is filled without opening the bleeder you could request the bleeder valve be opened, as it should be and see how much more you get into the tank. Might not be but a gallon, if that much, but when your Winter camping that gallon could make for some good sleep.

Thanks Jim.

It is obvious by my comments, 2 of the tank refillers in my area do this different with the bleeder. And I'm not saying what they are doing is right. Mainah triggered the thought process of the filling tank, the pump and the vaporization pressure going on in the middle to it. The pump itself does not have to do a lot of work. What do you recall doing with the bleeder when you refilled fork lift tank? See the questions I asked to mainah.

One learns or realizes something new every day on SOC!

Thanks

John

[JohnB]

Originally Posted by Jim-Bev-2363

Bits of information found online:

20# LP tank relief valves are set at 250PSI.

One gallon of propane liquid equals 270 gallons of LP vapor.

Pressures of LP at different temperatures: 70 degrees = 96 PSI. 100 degrees = 127 PSI. 110 degrees = 230 PSI.

I never looked up the relief valve setting, however 250psi could be correct. The high pressure tank pigtail hoses I believed are 300psi rated which would align with the 250 rating. And this should not be tripped on a regular basis.

The vaporization pressures you listed, I was finding theses slighting different. Be back on that later after I piece together the bleeder operation.

Thanks

John

[mainah]

We open the bleeder valve first off and allow it to spit a bit of liquid then shut it at that point it is full as it's going to get any more and the float will close the valve. As soon as the bleeder valve is closed the pump will stop pumping propane you can hear the pump fighting against the load. Yeah it's not a good ideal to be smoking around a propane filling station. The gas is not much of an issue as it dissipates quickly. We have found using that procedure the weight meets the 20# limit. The refill outfits fill by the gallon they just watch the meter on the pump and shut it off when they feel like it.

[Jim-Bev-2363]

JohnB, When I filled tanks I would open the vent valve about quarter of a turn. I do not have any proof but it did seem that with the vent open more, say about one full turn, the tank would fill faster, but that might just be in my mind. I did read somewhere that during the filling process the vented amount of gas was minimal. Remember that one gallon of LP equals 270 gallons of LP in the gas state so venting doesn't really waste much liquid gas, which is what we pay for.


Edited to add more online information:

"Propane Limits of Flammability - The lower and upper limits of flammability are the percentages of propane that must be present in an propane/air mixture. This means that between 2.15 and 9.6% of the total propane/air mixture must be propane in order for it to be combustible. If the mixture is 2% propane and 98% air, there will not be combustion. If the mixture 10% propane and 90% air, combustion will not occur. Any percentage of propane in a propane/air mixture between 2.15% and 9.6% will be sufficient for propane to burn. However, an improper air/gas mixture can produce Carbon Monoxide (CO) that is a deadly product of incomplete combustion.".

Found that here: http://www.propane101.com/aboutpropane.htm