Original article → Why fire trucks break down
Author(s): Chris Dennis
Published April 22, 2013 | From Firefighting in Canada
The training division of the Vaughan Fire Rescue Service provides driver training and pump operator certifications for all of its firefighters. During the last pump course, one of our training officers asked for an emergency vehicle technician (EVT) to attend the training to check and report on a water leak from the pump with which they were training. The truck in this case is a 2002 Spartan 17-metre aerial with a Waterous 6,000-litre-per-minute (LPM) pump. It was determined that the truck was leaking water from the pump packing area. The technician also picked up on a tinny rattling noise while in pump gear.
We removed the truck as a training vehicle until we could get a handle on what was leaking and what was causing the noise. In the shop, we raised the truck and engaged the pump.
The input and output shaft seals in this pump are mechanical seals. Any drastic change in temperature will damage these seals. Each end of the shaft, as it exits the pump, goes through a seal or packing assembly inside a unit we call the stuffer box. We found that the front mechanical seal in the stuffer box was badly leaking, and quickly saw that the seal had shattered (see photo 1).
The stuffer box got its name a long time ago, as it was once stuffed full of grease, before eventually becoming the product we have today. This sealing area keeps water inside the pump and keeps air out. Because the shaft typically spins at road speed, it needs to be cooled. The friction buildup in the shaft-to-seal area of our truck was immense.
In the early days of the stuffer box, the grease would melt and ooze out. It was the job of a firefighter to repack this stuffer box with more grease so that the water would stay in the pump. Sometimes even horse hair and grease mixed together would be stuffed into this area to keep the water in. The cooling down of this area became a problem until someone figured that if the stuffer box was drilled so that the water could get to the packing before it overheated, it might stop the seals from burning away. It worked. I am not sure in what century this was discovered, but I have worked on a 1919 American LeFrance pumper ladder and the fire pump looked the same then as it does today – only the pumps today are bigger.
So, this brings us to our overheated pump. You might wonder how, in this day and age, a fire pump can overheat, especially when water is our main source of cooling and extinguishing a heated source.
Imagine you’re drafting a large volume of water from a pond or other source, and the incident commander asks for more water. You check the ULC plate for flow ratings and then check the gauges (for more information on the ULC plate and the pump panel, see my column in the November 2011 issue of Fire Fighting in Canada). However, when drafting, the compound intake gauge is the only gauge able to read either negative or positive pressure. In a draft, we are reading negative pressure with a minimum of -55 centimetres (-22 inches) of vacuum at the maximum-rated flow of the pump. This means that, if we go below these numbers, the pump is no longer able to bring in the appropriate volume of water to get through the eye of the impeller, or rotor, and out the discharge side to the hose lines. We have put to a higher demand on the pump in order to move more water. All we are doing is building more pressure, though we are not moving volumes of water.
So, as this is happening, heat is being generated at the intake side of the pump – so much so that, at 100 C (212 F), the water starts to boil.
As we try to overwork the pump, it gets hotter, turning water droplets at the intake side into steam. According to Wikipedia, the pressure on the liquid causes the formation and then the immediate implosion of cavities, or tiny bubbles, in the liquid. When this happens, you can hear what sounds like marbles rattling around. This is called cavitation. Temperatures are well over 93 C (200 F) when this process is occurring. Now we have overheating.
This all comes back to a better understanding of what’s happening inside the truck. Our training division trains our pump operators on this process. But, if you don’t know, ask. If you’re still not sure, ask again; and, if it still doesn’t sink in, go back to the basics and start over.
At this point, you have been flowing water for some time. You have heard this noise but thought nothing of it. You remembered that the pump cooler valve should be opened and when you reached past the intake steamer port you could feel the heat off of the steamer connection (see photo 2).
This is an important area to be checking during any pumping evolution. It tells a story. If the steamer connection is getting too hot, it means that the water is boiling and causing damage. If this is the case, open the cooler and, when you return to the pump panel, recheck the ULC plate and the gauges, and you may determine that your pump is running at -38 centimetres (-15 inches) of vacuum and is cavitating. A lot of heat has been created and the pump cannot keep up with the demand. So, you notify command that you would like to cut back by 200 kilopascals (kPa) to be more effective. Command will likely approve the request, so you throttle back, do the adjustments and instantly the pump cools down and begins to draw in the right amount of cool water.
But, what happens when you boil something and hit it or quench it with cold water? If you were to take a glass and heat it until it is over the boiling point, then instantly hit it with cold water, the odds are that the glass would shatter. A mechanical pump shaft seal in a stuffer box is much like the glass – if you heat it then hit it with cold water, it will shatter (see photo 1).
Overheating a pump
Listed below are a few things that can contribute to the overheating of a pump, which leads to the pump’s failure:
- Running the pump dry of water with the static tank closed to the pump
- Running the pump dry of water with the pressurized source intake valve closed
- Recirculating water faster than what the tank-to-pump valve can achieve (a.k.a. cavitation)
- Recirculating water for long periods of time at excessive revolutions-per-minute (RPM) rates
- Dead-heading the pump (i.e., when the water is under extreme pressure but cannot go anywhere, so the pressure inside the pump builds)
- Cavitation during drafting operations
Keep in mind that, when a mechanical seal leaks water, it will not assist in the draft. Atmospheric pressure within the pump housing is evacuated when the primer is pulled. The water in the suction pipe is pushed up into the pump as the atmospheric pressure outside the pump is now greater than it is inside the pump. Creating a watertight seal in the plumbing, the pipe is then able to pick up that water and draw it into the pump. If the seal is damaged and vented to atmosphere pressure levels, the pump will never create enough of a vacuum to draw the water. See NFPA 1911 for specs of recommended water drip and manufacturer recommended drip rates.
The Waterous pump we are working on has a failed mechanical seal due to overheating. The seal shattered when cold water was introduced. We determined that the pump will not prime since the seal is broken, and we still hear a noise. It’s time for failure analysis: we want to pull on the primer to see how many centimetres of vacuum we can pull down and for how long it will hold (refer to the NFPA 1911 pump test specs). This will tell us if a valve is leaking.
Remove all the caps, one at a time, and listen to the ports. It was at this time that we found a piece of brass in the end of a 65-millimetre discharge valve on the officer’s side. This is not good. We removed the large steamer caps on both sides, as well as the ionic screens. Both screens, as well as the attached screen fins, showed signs of erosion. The screen, in addition to stopping debris, also breaks the stream and takes the static charge out of the water. The static charge can cause a galvanic reaction between dissimilar metals and, in turn, cause corrosion. When you are doing pump checks, be sure to remove these screens and check their backsides – this is where they first erode.
When they become weak, they stop less debris and the fins can easily break away when hit. Our technician looked inside the steamer port horns (see photo 2) and down to the impeller. We used a snake scope with a digital viewing monitor so we could see inside the pump. By moving the scope from side to side, you will be able to see not only the eye of the impeller but also the stuffer boxes at both ends, as well as their rudders (see photo 3). You can see by the photos that the rudders are missing some of their leading edges. When inspecting a pump, these are items that can break away and then move into the pump. We then removed the drain plug in the bottom of the impeller housing; this gave us a direct view of the discharge side of the impeller, which was missing most of its fins (see photo 4).
The stripper edge of the pump housing, or casing, was broken away (see photo 5). This is the side of the impeller out of which the water flows. The water is squeezed into this area, creating the high pressure needed to move the water. When you place your thumb over the edge of your garden hose, you create a narrow opening that backs the water up in the hose (or, in this case, the fire pump), creating large amounts of pressure and coming out with great velocity. With this portion blocked, the pump’s performance is immensely affected. How did this happen? We had already found that the pump was overheating and we believe that cavitations were causing this. Well, this truck would have to have been drafting its whole life and been pumped poorly the whole time, and, even then, this might not have happened.
This truck has been in a couple of different fire stations in its day, however, all with hydrant sources and very few non-hydrant areas. You can see the porous areas in photos 3 and 6 where cast-iron products had been taken away. It is evident on both the intake and discharge sides of the pump. Forensic findings determined that this pump had been overheated on many occasions. When you put excessive heat to concrete, it pops and breaks away. This is called spalling (see photo 6). The same thing has happened to the cast-iron products. I am not a metal expert, but, like concrete, cast iron is made with many dissimilar products, usually iron, carbon and silicon. These products, when mixed together, never perfectly interlock, so there remains an air gap. When the air is superheated, it expands, pushing against the solid product, forcing it away. It then pops out and breaks away. This is what happened to the stripper edge of our pump. It is one of the thinnest areas of the pump and is often under the most stress. So, if you heat it until spalling occurs, the thinner areas will break away. The stripper edge broke off and fell into the discharge side, causing a catastrophic failure when it jammed itself up with the fins and the casting.
This process of heating the pump can be corrected. See the previous list regarding the causes of overheating.
Instruct your pump operators and remind yourself to:
- Be sure to open the tank-to-pump valve.
- When hooked to a pressurized source, open the intake valve all the way (like a hydrant).
- Listen when recirculating from the pump to the tank. You can outpump the tank-to-pump valve, just like cavitating.
- When recirculating the water, be sure to monitor the steamer connection for heat buildup. Give the water a place to go other than back into the tank.
- If no water is flowing, be sure that the pump does not stay dead-headed for long periods of time. Give the water a place to go, but do it safely. You don’t need ice buildup in your work area or more water damage inside the property you have just protected.
- When drafting, pay close attention to compound gauges and do not go below suggested pump ratings.
Water is the best means of keeping the pumps cool. Remember to add a fresh source of water into the pump gradually. Give
the water a place to go so that it does not overflow. During the colder months of the year, throw a second line off into the ditch or away from your work area and allow the pump to move some water out.
Be safe always and remember, rubber side down.
Chris Dennis is the chief mechanical officer for Vaughan Fire & Rescue Services in Ontario. He can be reached at Chris.Dennis@vaughan.ca