Tips for Automatic Pump Pressure Governors
By Mike Laskaris
Technology is a great enabler. Everyone knows that! It makes our lives easier and more productive in so many ways that we can forget that new technology often carries with it a burden of new training requirements for the Fire Service. It may be easier to recognize the obvious hazards in an air bag system, hydrogen powered vehicle, or a high pressure gas cylinder, but there are training issues with many other common technologies as well. This includes the equipment we specify, purchase and use every day.
This article will focus on some realistic operational situations that can occur with Automatic Pump Pressure Governors (APPG). There are simple reasons why these issues can occur and training measures and guidelines that can be put in place to help avoid potential problems. Relief Valve systems have their own set of limitations and are really a separate subject.
Effective engine company operations require consistent pump operation and modern automatic pump pressure governors provide excellent control over pump pressures in a variety of situations. However, as Fire Service equipment has improved over the years, some operating characteristics have changed. Understanding and training with the pump pressure governors as part of a robust training program helps ensure these devices are part of a technology solution, not a new problem.
Why is an APPG needed? Or, why is a standard relief valve needed for that matter? These questions should already be answered in every pump operator training program.
- NFPA standard 1901 says that each pumper shall have a “pressure control system.” Why? Because pressure control should be an integral safety feature to the fire pump system. For our purposes here, a “fire pump system” includes the fire pump and its discharge outlets.
- The purpose of the discharge pressure control system is to protect fire service personnel. It does this by preventing dangerous and/or excessive pressure rise. This prevents injuries to personnel who are holding attack hoses and anyone in the vicinity of a discharge hose that could be compromised by a sudden extreme pressure rise. The appendix of NFPA Standard 1901 has some additional information on pressure controls.
Today, with the modern diesel engine, the two most widely used pump pressure controls are the internal relief valve and/or the APPG. Many firefighters have been trained on and are accustomed to using internal relief valves. With APPGs becoming more and more popular, this is presenting a challenge in that many firefighters may not understand the differences and characteristics of the Automatic Pump Pressure Governor.
Pump operators need to understand that modern APPG equipped pumpers have different operational characteristics than internal relief valves and may be different than the older governors in service. While these devices all meet the fire service need for discharge pressure control, they are designed to work differently and they have different operational strengths and weaknesses.
The newest NFPA 1901 standard requires a pump pressure control system much like the older standards. The pump pressure governor meets this requirement by changing the engine speed to control pump discharge pressure. Modern engines have electronic controls and the pressure governor “throttles” the engine speed up or down to maintain the pressure set by the operator. Different brands of pressure governors on the market today may have different appearance or features, however they all have the same basic core function: to maintain a constant discharge pressure by controlling engine speed.
Almost all modern Fire Apparatus around the world utilize centrifugal fire pumps. These pumps have many advantages over other pumps that were tried in the distant past. Centrifugal pumps will have a characteristic curve that defines how the pump will perform. The pump normally operates on this curve. Each line shown in figure 1 is for a single engine speed. You can see how as flow increases the pressure goes down for the same engine speed. While you can draw the curve differently, the action of the pump is the same. This is determined by the physics of centrifugal pumps and is not affected by brand of pump or size. Different pumps may have a curve that is shaped a little different, but these general concepts apply.
As discharges are opened and flow is increased, the pressure governor, in pressure control mode, will change the engine speed to keep the pressure the same. When flow is decreased by closing discharges, the automatic pressure governor reacts by reducing the engine speed to keep the discharge pressure from increasing beyond the NFPA limits. In fact, most pressure governors on the market today far exceed the minimum NFPA requirements with much better pressure control than required. Modern, reliable electronics make this possible.
The operator can select between rpm and pressure control modes. Most operators use pressure control mode when they are pumping to effectively control the pump discharge pressure. Switch to rpm mode, and the operator is controlling the engine speed directly. The pump discharge pressure is not actually controlled in this mode on most governors in service. (Note: Some of the latest generation governors actually prevent pressure rises above a certain limit even in rpm mode. But this is a relatively new development, so many existing systems will simply hold the pre-set engine speed and the pressure can follow the pump curve wherever it leads.)
Training Example 1
To give a good training example, use rpm mode to establish a common operating pressure of 150 psi operating from the on-board water tank. Next hit a hydrant or supply hose with a residual of 75 psi. What will the resulting pressure be? With most governors in service today the resulting pressure will be 225 psi. What will happen with the equipment you have in service? Does it have a pressure control limit to limit the pressure increase even in rpm mode?
As mentioned earlier, some of the latest Governors on the marketplace will hold the pressure to less than 200 psi in this training evolution because they monitor pressure even in rpm mode and act to keep pressures within “safe” limits. Are all your operators aware of this feature on newer equipment? Do they even run the system in rpm mode in training to find out the characteristics of this mode? Even if your department guidelines call for operating in pressure mode most of the time, make sure your operators are familiar with the rpm mode of operation. If they need to use rpm mode on the fireground for any reason, an emergency response shouldn’t be the time to learn about it.
Why do we need an rpm mode? The rpm mode will allow the pumper to continue to operate if a pressure sensor clogs or fails and will allow constant engine speed operation for certain types of equipment like some types of CAFS or other engine driven equipment that need a relatively fixed speed for best performance.
Typical pressure governor panel showing mode selector
and integrated message center. This model also has a
preset button built into the panel that allows the operator
to run the unit up to a pre-set pressure with one button.
It is an excellent idea to place a sticker on the panel showing
The pre-set pressure for that apparatus to avoid possible
surprises in actual operations.
A governor in pressure mode maintains the pump discharge pressure as long as the pump has an adequate supply of water, and is within the operational engine speed range. Obviously the engine has a maximum speed and a minimum speed. If the operator needs a 100 psi discharge and the normal “idle” pressure on the engine is 50 psi, there is no issue when operating from the on-board water tank. However when the pumper is on a 100 psi hydrant, then the minimum discharge pressure will be 150 psi no matter what mode the governor is in. The residual inlet pressure is simply added to the net pump pressure to get the resulting discharge pressure.
If the water supply is limited and the pump enters cavitation, the governor will attempt to maintain pressure by increasing engine speed. The operator must recognize the situations and signs of cavitation to avoid potential problems. A pressure spike can be generated when a pump pressure governor equipped pumper is placed in a marginal cavitation situation as seen in example 2 below.
Training Example 2
Lets explore a possible pressure spike further thru an example: A Fire Department Engine begins operations from the water tank at 150 psi in pressure control mode. As the 75 psi supply line is charged, all the air is not bled from the supply hose. (An easy error to make.) The intake valve is opened and the residual air from the hose is pushed into the pump. As the pump becomes air bound and the impeller cavitates, the discharge pressure goes down and the governor does its job, increasing engine speed to maximum rpm to try and maintain pressure. This engine speed may be equivalent to 300 psi or more, yet the pump is only discharging 150 psi (or less) due to the air in the pump. At this point the pump is not running on the characteristic curve in figure 1. The pump is “off the map” because it was designed to pump water, not air. As the water supply pushes the air out of the pump discharge, the impeller is hit with the water at the supply pressure of 75 psi. Doing the addition in your head, you probably realize that a 375 pressure spike may be generated from this pumper in this situation. It doesn’t last long as the governor returns to lower engine speeds as soon as the high pressure is detected. However, it does not take long for that pressure spike to do some damage. If this was a two stage pump, and the pump was in “series” or “pressure” mode, the pressure could spike to 675 psi! This is obviously a problem for the nozzleman and the hose line team.
Are your operators trained to make sure all air is out of any supply lines when changing from tank to positive pressure supply lines? Do you have SOGs that call for making the tank to supply line change in rpm mode to avoid such a problem? Doing this requires the operator to slowly open the intake valve while reducing the engine rpm manually to maintain pressure. Run this evolution at a pumping drill using lower “safe” pressures and make sure your personnel understand these issues. Establish your operational guidelines to match your equipment characteristics.
Many APPGs on the market today have various forms of protection from out of water or cavitation situations as described above. However different engine combinations do not all react the same and a slower responding engine and governor combination could allow these pressure spikes to occur while a different combination could be relatively immune to this problem. The pressure in the pump discharge may not have deteriorated enough to allow the governor’s “brain” to figure out that there is a problem. Sometimes it’s simply a matter of timing, the positive pressure water has to get to the pump at exactly the wrong moment to create a problem. It can and does happen. We still need to be smarter than our equipment and avoid these problem situations. If the engine speed is increasing and the pressure is not, the operator needs to know there is a problem and intervene by entering rpm mode and lowering the engine speed while restoring the water supply.
Having run this evolution with different Engine Companies with different brands of pumps and governors, I can tell you that not all governor / engine / pump combinations react the same. Some engines react more slowly to changes and two stage pumps in series or pressure mode obviously can produce more pressure at maximum engine speed for a higher pressure spike. In addition, a numerically higher (taller) pump gear ratio gives the pump additional pressure capability which may lead to a larger pressure spike in these situations. We have even run into pumpers where the engine and governor were so quick that we could not induce a pressure spike in training, despite numerous attempts. Do your operators know the how their pumper will act in this situation? More importantly, do they have the knowledge and training to avoid the problems from occurring in the first place? In departments where an operator can run any one of a number of pumpers on any given call, is there any common operating characteristic between the available apparatus? Two engines that look identical may have vastly different operating characteristics, due to the exact equipment and age of the equipment.
Similar problems in pressure governor operation can occur when running large caliber streams. If the water supply can not keep up with the required flow and residual inlet pressure drops low enough for cavitation or even collapse of the supply lines then the pressure governor will again increase engine speed in an attempt to re-establish the discharge pressure. Many governors will run several seconds at full throttle before they recognize cavitation and slow the engine down. If a discharge is gated or the supply is improved while the engine is at the higher speed, the pump can come out of cavitation instantly and a pressure spike results. Do your operators know the warning signs of cavitation? Will they know what to do to avoid problems?
Does this seem like a new problem? So why wasn’t this a problem for pump pressure governors from years ago? Ironically, the improved modern electronic engines with increased torque as well as the availability of LDH supply lines and larger pumps, have given the typical pumper much more water handling capability then ever before. This big water capability can darken much more fire, but the ability of the engine to run to high speeds under load is part of that system. On an older 225 hp mechanically governed pumper, there was much looser pressure control and the engine simply would not support the rpm response that helps the modern apparatus supply water more consistently. At the same time the older apparatus were not able to create pressure spikes due to the slow response and poor operation of the governors available. The modern APPG equipped apparatus provide much better control as long as the operator has the training to avoid problem areas.
Some pressure governors have the
engine information integrated onto the display.
Different brands of APPGshave the same basic operations. Note the pre-set button here.
Remember that pump pressure governors come in different brands and each has their own operating characteristics when combined with different engines and pumps. It is not just the brand of governor, it is the combination with the engine that can make a difference. Make sure your operators know how your equipment will perform in unusual or error-type situations before they happen and have the procedures and training in place to prevent problems before they start. It may be wise to ensure that some older apparatus that still have a long service life ahead of them receive an APPG upgrade. In this way the APPG operational characteristics can be more uniform across the department and training can be standardized. Contact the manufacturer or dealer to determine if an upgrade for your apparatus is available. This is one way to make sure that all your apparatus will share common operating characteristics.
Modern electronics can simplify pump panels by combining controls and
indicators into one integrated panel. This Command Master panel from
Class1 has foam and engine information as well as APPG controls in one
unit which can greatly simplify the pump panel.
Automatic Pump Pressure Governors can enhance the operation of FD pumpers, but the training is never automatic. With a little effort, operator training evolutions involving the operation limits of your governor as well as possible errors, can be added to the regular training regimen, avoiding surprises on the fireground. Work some pumping evolutions into your training that first benchmark how your equipment responds, and then places adequate training and guidelines in place. Make sure these training evolutions are conducted at lower safe pressures to determine how that particular engine will respond. Smooth and safe engine company operations revolve around the basics of supplying water reliably in a wide variety of situations. Take advantage of all that technology has to offer to avoid problems and don’t let technology stop you.
Some wise soul once said looks can be deceiving. That can be said for Fire Apparatus Hand Throttles as well as many other things. I can recall when all hand throttles on Fire Apparatus turned counterclockwise (CCW) to increase speed. When a rookie first encounters training, it may seem odd as the volume control on a radio or a light dimmer in a house turns clockwise (CW) for increase. But all fire apparatus once had CCW hand throttles. Or so it used to be. Perhaps it is normal to everyone else but now hand throttles come in clockwise and counterclockwise. An engine company with two late model pumpers can have different rotations for each throttle. This can lead to some interesting commentary when the experienced operator/engineer can not get water pressure out of the new rig.
Sure, there is a clearly identified arrow on the throttle that turns clockwise, but at 2AM with fire showing and adrenaline pumping you will swear its not there. It is amazing how long an experienced and effective operator can be stuck on such an issue when moved to a new or backup engine w/o enough practice at the pump panel.
People who make their living in industry improving the quality of services and products often look for ways to reduce the chance of errors. Quality systems and programs go to great expense to eliminate chances for error. They call this “error proofing”. Keeping your controls consistent is an easy way to reduce the chances of error in emergency situations.
Having otherwise identical controls on engines with backwards throttles is asking for delays and errors in water supply and support of engine company operations. Clockwise or Counterclockwise, it really doesn’t matter which way you throttles turn as long as you are consistent and proficient. Make sure your engines have consistent and familiar controls. Luckily, most modern hand throttles are electronic and can be easily changed so you can make the fleet match relatively easily. Don’t forget to check this on a new pumper inspection or better yet put it in your specs to avoid the problem before it starts.
Michael A. Laskaris, PE is the Director of Engineering for Hale Products Fire Suppression Division in Conshohocken, Pa. He is a State Licensed Engineer in Pennsylvania and a Volunteer Firefighter with over 20 years of experience. He has been awarded both US and foreign patents for products related to the Fire and Emergency Services.