Wildland Engine Cooler Melt Down
Keep the Engine Cooler Valves Closed
In this report, I and another NV-xxx(Eng Capt), look at a problem believed to be caused by an Engine Cooler Valve being left in the OPEN position continuously at the direction of recommendations (as stated to myself) and how this can create a very serious personal safety or injury risk, that is completely avoidable. This is likened to and nearly setting up a duplication of the 2017 Crescent Fire Scald Injury that took place on the Plumas NF during a parallel Pumping operation that lead to a failure of a coupling due to the high temperature/pressure combination. Or like the Little Horse Creek Fire pump incident this year (2023). In this case, three engines are experiencing the same issue and are NOT even engaged in a pumping operation. The severity of this is such that if the apparatus was continuously operated in this fashion and then plumbed into a hose-lay, the pump motor throttled up to the same or similar pressures as the 2017 report indicates, there would be no warning to the operator and it could lead to near-instantaneous failure upon reaching whatever pressure would be necessary for a hose to separate from the already hot expanded hose fitting!
So first there are few disclaimers,
That these are my initial findings and my own mathematical investigation into a problem based upon what the individual reported/asked of myself while I was out on a taskforce roll.
This is not an indictment against anyone.
This information is new as of this year (late) 2022, so I seriously doubt anyone has had a chance to even review this information as of yet due to the time of the year we are in so keep that in mind.
My statements made herein are my own and are done to show mathematically that the temperatures and time frames that were said to have taken place actually could have taken place as this person believed and the potential dangers of those aspects is revealed.
I had a very interesting question presented to myself during the 2022 Taskforce roll in Prineville, Oregon that had proven to be quite interesting and generated a lot of thought on several parties.
The question that I was presented with was: “What would cause an engine's tank water to become hot as you would have for hot tea”? My first impression was okay this is some kind of a trick question and I would just wait for the punch line. However, they were extremely serious. There was a definite problem with an engine (as in apparatus) heating tank water (not known how at first) and at first, all of the details and facts were not instantly conveyed to me, so I had to offer up statements and then ask some questions and offer up more statements, etc. The story is explained as follows:
An engine from the xxx District BLM was said to be experiencing a problem with generating some exceptionally high discharge temperatures. The exact temperature was not known but we can be pretty accurate in our estimation of this temperature and this will be disclosed shortly. It was stated that the Engines Auxiliary pump motor would be started and throttled up to approximately 1800 Engine Rpm and then the pump to tank valve would be opened along with the recirc valve. The pump was said to be having no issues with flow or pressure.
The initial thought process was that the area of concern would only be either a blockage in the discharge path back to the tank OR a problem with the Heat Exchanger. The question was presented to the Engine Captain about the quality of the Tank water. It was asked if there was any discoloration or coolant that could be seen inside the tank water or during the discharge. The response was “No, there is nothing in the tank water other than clear straight water, no discoloration, no odor, nothing”.
After being told multiple times that the pump-to-tank valve was open and working correctly and that there was nothing wrong with the impeller, the focus again returned to the Heat Exchanger. It was suggested to do a couple of tests and make sure that there was no blockage in the discharge valves and further urged to take the pump to tank valve apart to physically inspect for any return path blockage. At this point is when the decision from the local leadership decided to take the engine to xxxx in Utah. (this was due to the apparatus was still under warranty).
After a while, contact was again made with the Engine Captain to see if new information had been determined. It was stated that at first “xxxx could not duplicate the problem”. They were informed by the captain on how to set the apparatus up in the exact conditions to which the Engine Captain uses the Apparatus. After several hours the Engine Captain received a call from xxxx indicating that the problem had presented itself and “they'd have to dismantle some things to determine the cause”. Later they told the Engine Captain that the impeller was distorted and this was the cause of the problem.
Upon the Engine Captain telling me this, he also told me that two other engines were experiencing the same issue. A second xxxx District Engine and an bbbb District Engine. I had asked them if the engines were all of the same make and model pump, the apparatus chassis is not relevant at this point. I was told yes.
Upon learning that it was not isolated to simply one apparatus, but rather three apparatus changed the focus to now this is highly unlikely to be a problem with a “distorted impeller” because the likelihood of having 3 distorted impellers in 3 separate engines from the same manufacturer that has to be physically inspected before installation by a person is not just unlikely, it is practically astronomical. This was ruled out as “the” cause. Secondly, to distort a bronze impeller its temperature would have to be first raised to nearly the same temperature as that of the casting temperature, which is well over 1,000 degrees. Now while a distorted impeller “could” somehow be the cause, I personally do not believe such is the case. At this point I told the Engine Captain if the pump impeller is distorted, you want that impeller shipped back to you for personal observation/inspection.
I had stated to the captain that something caused this impeller to be distorted first (Assuming this is true. I again do not believe it to be the case). This then forces one to look back at the operational aspects as a possible cause. I asked how the valves were configured during this specific instance.
Aux Pump Motor - ON – Running to 1800 Engine Rpm
Set the Tank to pump – OPEN
Set Pump to Tank - OPEN
Set Engine Cooler - OPEN
Set Pump Bypass - OPEN
All other Discharges - CLOSED (only opened those line(s) as needed then Shut Off/Closed.
I instantly noticed a problem here. The Engine Cooler line is OPEN. I asked if this valve was left open all the time or closed. I was told it is left open because that is what NIFC says they (all engine operators) are to do. I told him that by doing so you are pulling the hot water temperature from your auxiliary pump motor and transferring that 180 to 195-degree coolant temperature into your engine's pump and then recirculating that hot water into your tank. This is the cause of the high temperatures and not the pump impeller. Whatever the thermostat temperature rating is set to open, is the same temperature that will be pulled off and transferred into your apparatus water tank.
Further, this now presents an exceptionally serious safety hazard (re-visit the Crescent Fire Scald Injury report from 2017, Crescent Fire Scald Injury 2017 USFS) and one that NIFC/NFEP should adopt ASAP or at the minimum, every district should instruct folks on CLOSING these Engine Cooler valves and “only opening them when drafting or pumping from a hydrant with cold water, or from a cold creek or lake”.
The engine cooler valve is a HEAT EXCHANGER valve. It DOES NOT AND WILL NOT keep the pump cool at all! The sole purpose of this valve is to keep the Auxiliary pump ENGINE COOL during operational periods of high ambient outside temperatures and little to no airflow. To make the auxiliary pump motor cooling system more effective, the engine coolant will pass through what is called a Shell and Tube heat exchanger.
Inside this shell are tubes and the tubes will usually carry the coolant and the shell will usually carry the cold water to carry away the high temperatures amplifying the radiator's ability to keep the Auxiliary Engine cool. The Heat Exchanger is just that, a Heat Exchanger, it exchanges high-temperature fluid for lower-temperature fluid and that fluid has to be placed somewhere. If however, this heat exchanger can only pull cool water from the Apparatus Tank, as opposed to being connected to a hydrant or a draft out of a cold river or lake, then the time that it takes to offset the cooler tank water to an equilibrium of the auxiliary pump motor temperature is a matter of hours.
On the end is the ½” NPT radiator connections and on the sides are the 3/8” NPT Engine Cooler lines that connect to the pump discharge and suction sides. According to Evanscoolant.com as one example, the water turns into steam at 212°F(sea level boiling temperature). Mixing traditional ethylene glycol antifreeze with water in a 50-50 ratio increases the boiling point to 223°F, which, is close to the operating temperature of an engine. Further, having a 14.5 PSI pressure radiator cap only further raises the theoretical boiling temperature to 253 degrees F. (No different than how a pressure cooker works).
So now we need to consider a few things right here: The thermal operating limits of fire hose couplings.
As shown in the image below, the Dura-Pak® hose has a temperature range of -40F to +150F, as most other fire hose fittings do. (most fire hose has nearly the same temperature ratings)
Next, we need to consider the Thermostat Temperature of the Auxiliary pump motor. As with most automobiles they usually range from 180F to 195F, some applications will use thermostats that open at 200 or 205 , but this is usually only done in severe and constant cold climates to keep the engine temperature up for better efficiency, etc. However, in our case, the weakest link is not the impeller or the heat exchanger. It is the HOSE fittings and their thermal ratings are nearly 30 degrees to 45 degrees “lower” than the standard operating temperature of nearly every motor driving a pump.
This brings us back to the 2017 Crescent Fire Scald Injury Report where two USFS type 3 Engines were parallel pumping to nearly 250psi each, creating a combination hydro-lock vs head pressure override and thereby deadheading a pump and producing no flow. Further, if these two engines ALSO had their engine cooler valve open all they would have done was exacerbate the issue and thereby reduce the amount of time it would take to heat the tank water temperature.
Let us look at the Engine Cooler valve line size. The heat exchanger shown above uses 3/8” NPT fittings to connect to the pump's discharge and suction sides. The lines are sized to allow the pump to recirculate water from the discharge, through the heat exchanger, stripping hot temperature away from the Auxiliary Pump Motor and pulling that into the pumps Suction or Intake side the intended purpose would be to have this hot temperature to be discharged out of a nozzle or hose line being mixed with substantially cooler water when operating from a draft or connected to a hydrant.
In the case before us, this is not being done and the cooler water is being pulled from the apparatus’ tank which is limited and will now be turned into a heat circulation pump of limited capacity.
We can therefore calculate the approximate time it will take to completely heat an 840-gallon water tank to 180 degrees using a 3/8” diameter line. Keep in mind that the size of the lines is calculated so that the pump is NOT going to lose pump discharge pressure for practical use. Below is how the estimations are determined based upon what the xxxx District Engine Captains' time frames were said to be, approximately anywhere from 2 ½ to 4 hours. We will look at this now.
Figures are based on 840 Gallon tank, a 50-degree water temperature inside the tank(est), and an altitude of 5,066 Feet MSL.
Boiling Temperature of water at 5,066 Ft MSL = 5,066/1000 = 5.066.
Now we take the Altitude decimal factor and multiply it by the lapse rate per 1,000 feet as shown.
5.066 x 1.84(lapse rate in °F per 1,000ft) = 9.32
212F Boiling Temp at Sea level – 9.32 = 202.68 Degrees F, Boiling Temp at 5,066feet.
The specific Btu (SBtu) of 840 Gallons of water is 202.68 – 50 = 152.68
The TOTAL Thermal Capacity in Btu of 840 Gallons of water (including Steaming) is:
152.68 + 970.3 = 1,122.98Btu.
Total weight of 840 Gallons is 840 x 8.34 = 7,005.6lbs
The thermal Capacity of 7,005.6 pounds of water is now 1,122.98 x 7,005.6 = 7,867,148.68 Btu. This is the total amount of heat that the Apparatus tank water can absorb for this altitude assuming the tank temperature is 50 Degrees F.
As shown in the image below, a 3/8” NPT line, flowing 3.5GPM the amount of heat that will be pulled off of a heat exchanger is 32,779.74 Btu per Minute.
If we take the total Btu capacity of the Apparatus Tank and divide this by the Exchange rate of 32,779.74 we’ll obtain an estimate of the time taken.
7,867,148.68 Btu / 32,779.78 = 240.0 minutes. Converting to hours: 240min / 60min/hr = 4.0 hours. This would have the tank temperature at the 180-degree mark.
202.68F – 180 = 22.68 degrees difference. If we ignore the steaming side (because you’re not making steam inside your tank) and simply want to take the tank temperature from the 50-degree range and raise it to the 180-degree range, then we could again figure this as in the same manner only you subtract the Steaming Btu’s from the initial Boiling Btu’s first.
180 thermostat – 50 tank temperature = 130 Degrees SBtu.
130SBtu x 840(x 8.34lbs.gal) Gallons total = 910,728 Specific Btu total. If we re-compute, we can determine that the 3.5 GPM on the 3/8” line would transfer this 180-degree heat to the apparatus tank as so:
3.5GPM x 8.34 = 29.19lbs/min
130SBtu x 29.19lbs/min = 3,794.7 Btu
910,728 / 3,794.7 = 240min or 240/60 = 4.0 hours.
The only thing here is that we are again not flashing to steam and are not able to generate the additional 970.3 Btu/lb that would be required to do so. We are only concerned about the temperature of the Auxiliary Engine Coolant. 180°F .
If we wanted we could determine that we only need to consider the time to raise the apparatus tank temperature to 150 degrees , (thermal/failure limit of the Fire Hose Couplings) then it would take even less time “pending impeller speed” & line size, 150 – 50 = 100SBtu.
However, this now lower temperature does NOT (in this case) change the time frame because we are still limited to 3.5GPM going through the heat exchanger via our 3/8” diameter line. To lower the time frames, we would need to move more water through the line, or increase the size of the Engine Cooler Valve Lines. Since swapping out lines is NOT practical or likely to be done by folks operating the engine, then the only logical factor is increased pump speed. And operating pumps with higher speeds actually happens continuously on Engines.
840Gallons x 8.34lbs/gal = 7,005.6lbs
100Specific Btu x 7,005.6 = 700,560SBtu
HEAT EXCHANGER LINE CAPACITY:
100 SBtu x 29.19 lbs/min = 2,919 Btu/min
700,560 / 2,919 = 240 min / 60min/hr = 4 hours.
The amount of heat that can be estimated could become exceptionally accurate if one studies the Pump Discharge pressure gauge or installed a line gauge with suitable adapters to measure the pressure drop. For example, in one of the xxxx Type 6 Engines, when opening the pump bypass line as an example (separate system from the heat exchanger valve) there is an approx. 9 to 10 psi drop. This would correspond to a flow between 12.7GPM and 14GPM. With a pressure of 9.06psi and 11.01psi respectively for the same size line.
If we figured that a 3/8” line will move 12.7 GPM through it then this corresponds to 105.918 lbs/min flowing through it. If we further multiply this by the specific Btu, for this case, it is 130 as before, then the 105.918 x 130 = 13,769.32 Btu/min
Using the original number of 910,728 / 13,769.32 = 66.142 minutes or 1.102 hours.
So by our estimations, we can now see that anywhere from as little as 66 minutes to 240 minutes (1hr – 4hrs) the tank water could be heated to the same temperature as that of the thermostat temperature in the Auxiliary Pump motor, which is instantly at a minimum of 30 degrees F over the thermal rating for the fire hose fittings we use.
Pressurizing a system with hot water only raises the boiling temperature of the liquid above that of the standard sea level temperature of 212 degrees.
For example: For every 1psi above 15.0psia atmospheric (called the Absolute pressure or PSIA), the temperature rises by 3 degrees F. If we have a System pressurized to 16psi the boiling temperature of the water is now raised to 216° . If this system is now pressurized to 20psia then the boiling temperature is raised to 228° Now, “most often” our discharge pressures are in the range of 150 to 200psig. If we say it's at 175psig, then this would be 189.7psia.
Then the Boiling temperature would be raised to nearly 382° Are you starting to understand the need for folks going to burn centers yet?
Calculating Absolute pressure (PSIA) from Gauge pressure (PSIG), take the gauge pressure and add 14.7 to it. For a gauge reading of 50psi add 14.7 to get 64.7psia. This new Boiling Temperature would then be raised to approx. 297° . This poses a very serious danger because the temperature, although well above boiling temperature for sea level, is still a liquid and is NOT yet boiling therefore the pump will NOT cavitate if it is in this state until it reaches 297°F. By this time, hose fittings and other components on the apparatus would have failed and under the high pump discharge pressures due to the temperatures, with personnel nearby, this would be a catastrophic failure.
This Initial Concern was brought to my attention by (no permission to use his name so withheld), NV-xxx Captain, and identification credit should be given to him. I have written this simply as a means to show mathematically how it is possible for the problems he described and experienced, to be taking place. He stated he went through all of the testing steps I had given for him to follow and test and after speaking with xxxxx (shop) believes that the initial concern(s) I handed out that the Heat Exchanger is the root cause/issue. I then after doing a follow-up conversation with him worked out these numbers here and concur with this determination and feel the above results speak for themselves and that all efforts should be made to affect a policy change immediately unless pumping from a draft with substantially colder temperatures or pumping from a hydrant, the Engine Cooler valve be left CLOSED!
It is recommended and believed that if we do not change this directive we will be putting engine operators and bystanders at risk of a scalding injury due to the temperatures as demonstrated being 30 to 45 degrees above the coupling thermal ratings and combined with the potential for high pressures in excess of 240psi would now be the equivalent of a blunt force object strike.
This information should be disseminated to all.
This is not a problem limited to only wildland apparatus, this is a inherent aspect in all fire apparatus that have heat exchangers.
Engineers Make it Happen!