I am working on becoming our corporate expert on Pressure Relief. I've read all the ASME & API standards.
We have a consultant who is recommending fusible plugs for tank jackets in a fire case to drain the liquid out of the jacket prior to it reaching its boiling point because the existing valves aren't sized for 2 phase flow.
I have several concerns about fusible plugs while I admit the valves are probably not large enough for 2 phase flow. They were sized for all vapor flow, which was the industry standard.
- Plugs aren't in the ASME Pressure Vessel Code
- You have to locate the plugs near the bottom of the vessel so if you have a fire in one room you could drain the cooling water for the entire building on to the floor of that room.
- Plugs have an unknown maintenance requirement. One website recommends replacing them every year. That would mean draining all of the cooling water system to do the replacements.
- Plugs can become corroded on the inside or outside and fail to operate.

I've talked to the manufacturer of the fusible plugs and they said Bayer and Dow use fusible plugs for this type of service.

What does anyone else think? Any experience with fusible plugs?

erobin,
As you are one of the corporate expert on Pressure Relief and you've read all the ASME & API standards. You should be one of the expert in this field...Allow me to drop a simple note before other experienced Che Jedi advise you...

The proposal looks to me is replacing the Ultimate Safeguarding device (i.e.PSV) which is mandatory in ASME requirement (i presumed the vessel jacket is designed to ASME) to other safety measures (i.e. : fusible plug loop)...If your application is offshore, fusible plug loop is one of the protective layer in additional to Ultimate Safeguarding device (USD)...and now this critical protective layer (USD) has been removed...i think another protective layer with same or even higher reliability, response time, SIL level, etc required to be in in order to convinced everybody that the system is "protected', specifically authority.

If this the way it will be implemented due to constrains, SIL level compliance and Risk & Consequence analysis may need to be conducted...

Just some personal thoughts...ignore if irrelevant...

JoeWong

@erobin:

I've been doing relief design for quite a number of years and I just started researching fusible plugs myself for use in the type of application you are discussing. For jacket protection during a fire scenario, the fusible plug appears to be a favorite means of protection by many members of the DIERS Users Group. From what I've gathered to date, you can treat them much like an orifice to calculate your flow requirements.

Before addressing some of your concerns, I do want to comment on the statement, "They were sized for all vapor flow, which was the industry standard." There is no industry standard in choosing the type of relief. I think you will agree that you are obligated to correctly identify the mechanism of your particular relieving scenario and not rely on what others do. Just because others may do it wrong doesn't allow you to follow the same incorrect assumptions.

1. The fact that it is not specifically mentioned in ASME does not bother me. After all, you are not using the fusible plug to prevent the over pressurization of the jacket, you are only using it to drain the jacket of the contained liquid before it can pressurize. Note that this may not necessarily be a good thing.

2. If I were to use a fusible plug in this application, I would strongly consider the use of an automatic shut-off valve in the line to the jacket in case of a sudden loss of header pressure, which one might expect to see if the plug failed. You can use a fire detector as well to activate the auto shut-off.

3. Just like anything else, I would go with the manufacturer's suggestion when it comes time for maintainance. You would not have to shut the entire coolant system down, just block in the jacket!

4. Corrosion may very well be an issue and use of a fusible plug is certainly not warranted where you know there will be a problem. However, the most common materials of construction for fusible plusgs are pretty good in cooling water and glycols. Discussion with the manufacturer is definitely required.

@Joe:

The use of the fusible plug is not a direct replacement for a PSV (or rupture disk). See my point #1 to @erobin above.

Phil,
This is bothering me. According to erobin, "We have a consultant who is recommending fusible plugs for tank jackets in a fire case to drain the liquid out of the jacket prior to it reaching its boiling point because the existing valves aren't sized for 2 phase flow."

My understanding is provision of other means i.e. fusible plug to detect fire and evacuate it content to avoid overpressure...This sound quite similar to providing a emergency blowdown system (BDV) with fire detection inplace and to evacuate it content to avoid potential of overpressure...To my knowledge, an ultimate safeguarding device e.g. PSV, RD, etc will still required eventhough emergency depressurization and fire detection system have been provided.

JoeWong

By using a fusible plug, you eliminate the need to worry about a boiling liquid that could cause a two-phase relief. Note I said "...boiling liquid that could cause a two-phase relief " because you will always leave just a little liquid at the bottom of the jacket. There should be enough disengagement space to allow all vapor venting at this point and @erobin said his PSV can handle this.

Yeah...Ok...the fusible plug loop, level valve (or BDV on liquid side), etc, the entire loop shall be "highly" reliable...

Just in case anyone is interested:

I've used thousands of fusible plugs in the past. I used them when I operated Acetylene generating and charging plants. The plugs have been standard compressed gas cylinder safety devices on Acetylene cylinders long before I came into this world - and that's a long time ago.

The fusible plug is sanctioned by American Gas Association and is a practical and effective way to safeguard against the possible explosion of Acetylene cylinders should they be exposed to a fire. I have seen such a cylinder caught up in a fire and actually expel its contents (as a flame) during the fire. It is very impressive - besides being effective. It works and prevents the explosive destruction of the cylinder and its accompanying danger to all in the vicinity.

Other compressed gas cylinders are also protected by fusible plugs (& rupture discs).

I thought I'd pass on this information in the event that someone hadn't heard about this "old fashioned" way of safeguarding equipment during a fire. The idea has been around a long time and it probably will never be replaced in the case of Acetylene cylinders - primarily because of its effectiveness.

P.S. - In case anyone doesn't know it, Acetylene cylinders contain the Acetylene gas dissolved in Acetone. The gas is not free in the cylinder. Therefore, you have a potential rapid evaporation of the Acetone + Acetylene evolution possible during a fire - the expelled fluids are 100% 2-phase in nature.