[Beowulf] bring back 2012?

Prentice Bisbal pbisbal at pppl.gov
Mon Aug 22 11:41:44 PDT 2016

On 08/22/2016 11:22 AM, Stu Midgley wrote:
>     While the risk of an explosion is a certainly a theoretical
>     possibility, In practice, the risk of this is virtually
>     non-existent for a variety of reasons.
>     With water, the processors and other heat-generating components
>     would fail from the heat before the boiling point of water is
>     reached, so there would be little to no generation of water vapor
>     that could lead to an explosion. Also, any heating/cooling system
>     with water would be designed to included an expansion tank to
>     account for the thermal expansion and contraction of water. There
>     are millions, if not billions, of homes and businesses in
>     existence with hot water heating systems, yet, I've never heard of
>     any of them ever exploding.
>     With Novec and other two-phase systems, the gas phase is
>     compressible, meaning it can store energy like a spring,
>     preventing or minimizing the case risk of an overpressure
>     situation rupturing the vessel. All that is required for this to
>     be used safely is an adequate volume for the gas, so that is has
>     excess 'capacity' to be compressed. This simple design is what
>     allows 20-pound propane tanks to be used all over America (and
>     probably other countries) to fuel gas grills and be left out in
>     direct sunlight all summer long, and be stored directly under the
>     heat-producing burners.  If those tanks were filled to the top,
>     they would explode in those conditions, but but leaving about 1/3
>     of the tank empty, the risk has been virtually eliminated. This
>     was actually a top we spent a lot of time discussing in my
>     Chemical Engineering Safety class in college.
>     This also applies to the tanks storing liquid nitrogen, liquid
>     oxygen, and other gases/liquid stored well below their boiling
>     point. Tanks of these substances can be found throughout the world
>     in industrial and laboratory environments, yet explosions caused
>     by them are quite rare. When they explode, it's usually because
>     someone who didn't know what they were doing overfilled the tank,
>     or the ambient temperature exceeded the designed safety margins
>     through some other catastrophic event. (structure file, etc).
>     Finally, all systems where this is a risk would have plenty of
>     safety features to prevent this. My gas water heater at home has a
>     simple temperature/pressure switch to safely discharge excess
>     pressure/temperature event. These are cheap, readily available
>     items that you can buy at any local hardware store. I also have a
>     steam heat system in my house. In the early days of steam heat, it
>     was not unheard for a steam boiler to explode with devastating
>     results, but just to some simple design elements (Hartford Loop)
>     and basic mechanisms (low water cut-off valve, pressure relief
>     valves) have virtually eliminated this risk.
>     Before I got in to HPC as a profession, I was a process control
>     systems engineer. My companies specialty was control systems for
>     boilers for power generation. The pressures of these systems were
>     much higher than what we're talking about here. Our systems had
>     plenty of pressure sensors, release valves and failsafes.
>     Incorporating any of these safety elements into a cooling system
>     like this is trivial, and I'm sure the vendors who sell such
>     solutions have already done that where appropriate
> But if you have 40kW of gear still running, your not storing the 
> liquid in the sealed container well below boiling point - its actually 
> the opposite you are running at or just above the boiling point.  Even 
> if you take the approach "our systems will shot down if we loose the 
> external cooling circuit)... that still takes time to recognise and 
> shutdown... mean while your systems are pumping heat into the tank.

Any failsafe mechanism would be designed to shut the system down with a 
wide safety margin.

> Again, with the boiler example, this isn't the sort of behaviour you 
> want in a computer room.  You don't want this stuff venting... and 
> also, try and get a permit to operate such a system in an existing or 
> new facility.

In reality, I don't think any of this stuff would need to vent, or even 
have vents, I was just explaining that there are a multitude of ways to 
mitigate the alleged explosion risks based on my Chem Eng. education and 
several years of experience as a practicing Chem E. I'm afraid I may 
have gotten a little carried away and distracted from the 'in actual 
practice' argument I was trying to make.

If you look at the IceTope literature, their Novec loops are designated 
'low pressure' so, I'm assume there is plenty of excess capacity in them 
for pressure to build up safely.
> With a non-phase change solution, this isn't an issue.
>>     Novec is a common fluid used in fire suppression systems in
>>     computer rooms... so you shouldn't have too much of an issue with
>>     the flammability of the fluid, but it is an entirely different
>>     issue to get them to warrant the solution.
>     If it's already used in fire suppressions systems, where it's
>     going to deliberately be sprayed into the atmosphere, I don't see
>     how a system where it's designed to be completely contained would
>     more of a safety issue, but this could be a local practice issue
>     (USA vs. Australia laws, etc.) In the event of an emergency
>     venting, as stated above, the vents are hooked up to exhaust
>     piping so that that gases released are piped away to a location to
>     where it can be safely released to the environment, so a venting
>     event would pose no risk to the occupants of the data center. This
>     is done all the time at chemical plants. In fact, venting like
>     this happens quite regularly in those environments, but I honestly
>     don't even see this type of venting being needed in a system like
>     this.
> If you look at the US laws around this (I've only been through the 
> Houston documentation - but I assume all states are roughly the same) 
> their is a massive different between holding a few hundred litres of 
> fluids (which is what is in a fire supression system) and say 30000L, 
> which is what you'll have in 30 tanks.
I don't think you'd anywhere near that volume of Novec in a system. The 
cooling systems volume are only partially filled with Novec, since you 
want to have a low pressure and room for the gas to expand. This system 
only works if the Novec can easily change state. Since the change of 
state absorbs a lot of heat, you need a lot less of the fluid to have 
the same thermal capacity.

Going back to my home's steam boiler example, My boiler only holds a 
couple of gallons of water. If I had hot-water radiators  or hot-water 
baseboards, I'd have you use a lot more water.

Also, the Novec is only in the primary loop. It condenses on another 
heat exchanger which transfers the heat to a secondary loop which 
carries the heat out of the data center, so not as much Novec would be 
need as you might be assuming.

 From what I recall, the Green Revolution system is a single loop filled 
with mineral oil all the way to the external heat exchangers.
>>     Perhaps the final nail in the coffin of the Novec solutions... it
>>     has been on many different booths at SC for many years... now go
>>     and try to find a vendor that will actually sell you a solution...
>     I guess you never stopped by the Icetope booth at SC, then.
>     They've had solutions on the market for several years now, and
>     have had booths at SC for several years now, too.
>     http://www.iceotope.com/
> I've seen them, I hadn't realised they were using Novec.  Even a 5min 
> look through their website doesn't make that clear.
Yeah, I noticed that when I checked the link, and I was very 
disappointed in that, since if I couldn't prove it, it wouldn't help my 
argument. They used to be much more 'vocal' about that. Icetope worked 
with University of Leeds to develop and test their system. If you 
remember, there was member of this list involved in that work who would 
occasionally post updates on their work here.
> Which brings us to another topic... and that's price.  The icetope 
> stuff looks very very custom... and thus very very expensive.  I can 
> purchase gigabyte or supermicro equipment already for the fluid we are 
> using (they modify the power supplies, leave the thermal paste off 
> components etc)... no modifications at our site necessary - and 
> relatively cheap (is any HPC gear cheap?)

Damn it! I can't argue this point at all. You are absolutely correct 
that Icetope's design requires purpose-built cases. I think with enough 
volume (no pun intended), the cases could be manufactured almost as 
cheap as existing air-cooled cases, but until they get that kind of 
volume, I'm sure they're more expensive another con is that I don't 
think any of the big OEMs (Dell, HP, etc) are working with them at the 
moment, limiting the options.

> -- 
> Dr Stuart Midgley
> sdm900 at sdm900.com <mailto:sdm900 at sdm900.com>

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