Sunday, February 9, 2020

Fire Pump Rating (Size) Selection

Fire pump sizing is not like commercial pump sizing. We don't care about efficiency, and you order pumps in only specific sizes. This article touches upon some items to consider when picking a fire pump.

In general, the first step is determining your system demand point. Discussion of how exactly you determine this is beyond the scope of this article and has a lot of nuance depending upon your site-specific needs. However, for simplicity let's assume that you have a dry-system in an attic with a demand point of 305 gpm (2535 sq ft x 0.10 gpm/sq ft x 1.20 overflow/imbalance factor).

Rated Flow (gpm) Sizing

The first item you must specify is the pump flow rate. Per NFPA 20 (2013 edition) table 4.8.2 pumps are only allowed to be listed with the following flow rates in gpm:

  • 25, 50, 100, 150, 200, 250, 300, 400, 450, 500, 750, 1000, 1250, 1500, 2000, 2500, 3000, 3500, 4000, 4500, 5000
So with our example demand of 305 gpm, would you go with a 300 or 400 gpm rated fire pump?

While manufacturer's pump curves go way beyond the ratings above, NFPA 20 section 4.8.1 only allows you to use a fire pump out to 150% of its rated capacity. That means that a 300 gpm pump can be run out to 400 gpm (300 x 1.5) and could easily meet the example demand flow rate. One could even use a 250 gpm pump with the corresponding max flow rate of 375 gpm (see the bottom of this article for additional discussion).

The other common condition is a standpipe. Assume you have two stair-towers and total demand of 750 gpm. Should you run a 500 gpm out 150% to 750 gpm? In our opinion that is probably a poor choice since you are so close to the 150% limitation.

So what is the harm in just selecting the next pump size up (i.e. 400 gpm for our 305 gpm example demand)? First, can you water supply handle flowing at least the 100% and/or 150% flow rate? While technically NFPA 20 allows you complete testing at less than 150%, if you can avoid this situation it would be better.

Second, there may be some hidden cost. For example going from a 400 to 500 gpm pump also means you go from 4-inch suction to 6-inch suction (see our NFPA 20 pump pipe size look-up app at https://anvil-fire.com/apps_pump_sizing.html). In addition, you are generally probably increasing the horsepower and therefore the overall cost of the equipment. Or even causing an electrical transformer to be upgraded.

Note that FM Global only allows you to run the pump out to 140% of rated pressure.

Rated Pressure (psi) Sizing

The second item you must specify is the rated pressure you want. Manufacturers must list each pressure rating in order to maintain listing with UL and FM Global. On occasion you will find some holes in approved pressure ratings, but AC Fire has a long history and therefore an amazingly wide range of rated pumps to accommodate almost any demand.

While not directly impacting your selection, it should be noted that NFPA limits manufacturers to how "steep" their curves can be in regard to pressure. See the curve below for the specifics. These limitations are a non-issue when you buy a listed pump as they have already had to comply with the limitations.

However, you should pay careful attention to the churn pressure (sometimes also called shut-off or dead-head pressure). When a fire pump is not flowing any water it produces the highest discharge pressure. In general most fire sprinkler components are listed to 175 psi.

Let's take the example of you have 50 psi static pressure from the city + our 100 psi rated pressure for a total 150 psi when flowing the 100% rated gpm. Sounds good, right? Maybe not if your churn pressure is 126 psi which is not unthinkable depending upon the specific pump selection (50 psi static + 126 psi churn = 176 psi). Always provide the max city pressure to your pump rep and look at the pump specific curve to ensure no surprises in the field.

In addition, make sure you check how much pressure you have at your desired flow rate from the city water supply. You can use the formula below or just use the quick calculator at https://anvil-fire.com/apps_log_graph.html





Check out AC Fire's online pump selector at https://www.selectacfirepump.com/ and play around with some selections, or reach out to us at sales@anvil-fire.com for support on your projects in the NorthWest.


A quick reference chart for the 150% and 140% flows of the various pump ratings.

Rated Capacity150%140%
2537.535.0
507570
100150140
150225210
200300280
250375350
300450420
400600560
450675630
500750700
7501,1251,050
1,0001,5001,400
1,2501,8751,750
1,5002,2502,100
2,0003,0002,800
2,5003,7503,500
3,0004,5004,200
3,5005,2504,900
4,0006,0005,600
4,5006,7506,300
5,0007,5007,000

Monday, December 9, 2019

Diesel Exhaust Requirements

The exhaust from a diesel engine driving a fire pump is an often-overlooked item.  The high heat put out by the exhaust is a hazard to occupants and can greatly contribute to overheating the room itself. NFPA 20 (2013 edition), section 11.5 covers "Engine Exhaust" and should be carefully reviewed. However as a starting point, the following items should be considered.

Exhaust Discharge Location

The first step is to determine where you are going to discharge the exhaust gases. NFPA 20 section 11.5.3 for "exhaust discharge location" has the generic common-sense requirements of do not discharge where you might hurt people, damage the building, or directed on combustible materials. But very little specific requirements are provided. There is a loose reference in NFPA 20 back to NFPA 37 (Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines). In addition, the International Mechanical Code (IMC) section 915.1 directs us back to NFPA 37 for guidance.

NFPA 37-2018 edition section 8.2.3.1 has the same generic recommendation that "Exhaust systems shall terminate outside the structure at a point where hot gases, sparks, or products of combustion will discharge to a safe location."

Not well referenced by the codes around exhaust, are the requirements for intake louvers.  These requirements provide more hard numbers that the exhaust sections.  The IMC section 401.4 requires intake louvers to be a minimum of 10-feet away from any hazardous or noxious sources (diesel exhaust would be considered noxious).
Example Exterior Exhaust Discharge - Not near any doors

Exhaust Sizing

Don't assume that just because the engine and muffler have a 4-inch connection, that you should run 4-inch all the way to the exterior.  Back-pressure on the engine due to excessive distances to the end of the exhaust pipe can cause poor engine performance. NFPA 20 states that the exhaust shall not be smaller than the engine outlet size and be as short as possible. NFPA 20 section A.11.5.2 further provides the generic guidance that if you have more than 15 linear feet (4.5 meter) of exhaust pipe, you should increase your exhaust one pipe size for each additional 5-feet (1.5 meters) of length.

However, rather than using this generic rule-of-thumb, we recommend consulting with the manufacturer.  Both Clarke and Cummins have some very easy to use calculators so that there is no question about exhaust sizing. Links to their web pages are provided below:

Exhaust Through Walls/Roofs

Obviously you don't want to put your 1200 F degree metal exhaust against wood studs and siding. NFPA 20-2013 section 11.5 provides the following guidance:
  • COMBUSTIBLE ROOF - Guarded at the point of passage through the combustible roof by ventilated metal thimbles that extend not less than 9-inches on each side (above and below) of roof construction and are at least 6-inch in diameter larger than the exhaust pipe or duct.
  • COMBUSTIBLE WALL - Metal ventilated thimble not less than 12-inches larger in diameter than the exhaust.
NFPA 37-2018 section 8.3 is also applicable for exhaust gas temperatures less than 1,400F (760C). Based upon a quick review of the data sheets for Clarke and Cummins, the typical exhaust temperature does not exceed 1,100 F. The requirements of NFPA 37 are basically the same as NFPA 20.
Diesel Exhaust Wall Thimble - Exterior

Diesel Exhaust Wall Thimble - Interior

Insulation

The exhaust pipe shall be covered with high temperature insulation or otherwise guarded to protect personnel from injury. This should also include the required minimum 12-inch flexible piece shall be installed between the engine and the exhaust piping (this piece is provided standard with all packages provided by Anvil Fire).

Diesel Engine Exhaust - Without Insulation

Diesel Engine Exhaust - With Insulation
Contact Anvil Fire (sales@anvil-fire.com) for support and sales of any product referenced in this article.

Sunday, August 6, 2017

New FM Global Fire Pump Marking Requirements

FM Global is updating their nameplate requirements for fire pumps. Nameplates for pumps manufactured after October 1, 2017 must now include the name of the pump driver and pump controller manufacturers and serial numbers per the sample below:

An example of a fire pump nameplate including driver and controller information

"With the global expansion of our business, we have seen a dramatic increase in the number of FM Approved fire pumps and pump packages worldwide," notes Dave Fuller, manager of FM Approvals' fire protection group. "This means that more and more manufacturers must rely on multiple facilities for distribution and shipping, and with that have come additional challenges to ensuring complete FM Approved pump packages are being supplied. By requiring more details be included on the pump or pump package label, we are increasing the transparency of this requirement to the end user. This creates a stronger paper trail and easier verification, and will help all parties involved should there ever be a question about a specific fire pump package."
Anvil Fire has seen instances of other distributors assembling different manufacturers together in their shop to save money. However, FM Global has always required that the serial number of the pump and controllers to be recorded at the factory. It is clear that FM Global is trying to further enforce this requirement.

Visit the FM Global web-page for additional information: http://www.fmapprovals.com/product-alerts-and-news-events/approved-product-news/approved-product-news-recent-issues/2017/apn-volume-33-issue-1/fire-pump

Friday, February 27, 2015

Diesel Engine Cooling Loops

Photo of Clarke Fire cooling loop name plate
Make sure you are in compliance with FM Global's requirements for cooling loops on diesel engines. FM Global Approval Standard 1333 was updated in 30 Nov 2013, but compliance was not required until 1 April 2015.

Clarke Fire has an excellent write-up on their web page that you can find here. But, we wanted to summarize the key requirements.


    Photo of new Clarke Fire cooling loop sensor locations
  • FM Approval - Cooling loops are now required to be approved and therefore they will have the FM logo on a nameplate.
  • Low Raw Water Flow Alarm - This switch is set to activate at 75% of the required flow rate.
  • High Raw Water Temperature Alarm - This switch is set at 105F (40C).

Wednesday, January 15, 2014

Installation of Diesel Fuel Tanks for Fire Pumps

After you have determined the size of fuel tank you need for a diesel fire pump, what are the general requirements for installation?  Assuming that you are under under the International Building/Fire Codes, you would go through the following chain of code references:

  1. IFC (2009 edition) 3401.2 Nonapplicability. This chapter shall not apply to liquids as otherwise provided in other laws or regulations or chapters of this code, including: ... (3) Storage and use of fuel oil in tanks and containers connected to oil-burning equipment. Such storage and use shall be in accordance with Section 603. For abandonment of fuel oil tanks, this chapter applies.
  2. IFC (2009 edition) 603.1 Installation. The installation of nonportable fuel gas appliances and systems shall comply the International Fuel Gas Code. The installation of all other fuel-fired appliances, other than internal combustion engines, oil lamps and portable devices such as blow torches, melting pots and weed burners, shall comply with this section and the International Mechanical Code.
  3. IMC (2009 edition) I915.1 General. The installation of liquid-fueled stationary internal combustion engines and gas turbines, including exhaust, fuel storage and piping, shall meet the requirements of NFPA 37.

So Fuel tanks need to comply with the following standards at minimum:
  1. UL 142 - Containment Products for Flammable and Combustible Liquids, Fixed and Stationary Storage Tanks, Special-purpose Tanks
  2. NFPA 20 - Standard for the Installation of Stationary Pumps for Fire Protection
  3. NFPA 37 - Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines
You will note that NFPA 30 (Flammable and Combustible Liquids Code) is not included on the list.  NFPA 20 appendix section A.11.4.3 clarifies that the intent is to apply NFPA 37 as the tanks is part of the "internal combustion engine system".  The specific explanation from the appendix language of NFPA 20 (A.11.4.3) is as follows:

Research has identified nothing in NFPA 30, Flammable and Combustible Liquids Code, or NFPA 37, Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines, that prohibits the outlet connection to the engine from the diesel tank from being in the location required by NFPA 20.
The applicable code is NFPA37, not NFPA30. The scope of NFPA 30 clearly states that if the installation meets the criteria in NFPA 37, then it satisfies the requirements of NFPA 30.
Therefore, NFPA 37 applies for the fuel tank for the fire pump, as it is considered to be part of the installation of the internal combustion engine. Subsection 6.3.2 of NFPA 37 deals with fuel tanks inside structures for fuels other than Class I liquids. Sections 6.6, 6.7, and 6.8 of NFPA 37 deal with filling, venting, and connections between the engine and the fuel tank, and these sections send the reader back to NFPA 30 for the requirements. A review of the tank chapter in NFPA 30 for fixed tanks with capacity of 119 gallons or more finds no requirement stating that the connection to the engine has to be from the top of the tank, if the tank is on the floor on legs, or otherwise above ground.

There are numerous requirements for the construction of a diesel fuel tank. However as long as you are buying a listed fuel tank, they will be integrated into the design.  So make sure that your fuel tank bears a permanent nameplate or marking the standard it was built to. It is not our intent to discuss how to build a tank.  This article is intended to address field installation issues.  In our opinion, there are four key items to consider:
  1. Location
  2. Venting 
  3. Fill Line 
  4. Containment  

Location

For most of the continental United States, fuel tanks should not be located outside due to temperature.  Diesel is a mix of hydrocarbons, and the components have different freezing points. For Number 2 diesel, as the ambient temperatures drop toward 32°F, it begins to cloud, due to the paraffin in the fuel solidifying. As the temperatures drop below 32°F, the molecules combine into solids, large enough to be stopped by the filter. This is known as the gel point, and generally occurs about 15 degrees F below the cloud point. 
11.4.3* Fuel Tank Supply Location.
...
11.4.3.2 In zones where freezing temperatures [32°F (0°C)] are possible, the fuel supply tanks shall be located in the pump room.
...
A.11.4.5 The pour point and cloud point should be at least 10°F (5.6°C) below the lowest expected fuel temperature.
Appendix section A.11.4.5 of NFPA 20 (2010 edition) states "The pour point and cloud point should be at least 10°F (5.6°C) below the lowest expected fuel temperature."  So per code if the temperature at your project site is ever expected to drop below 42-degrees you should not install the tank outside.  In practice we would not recommend tanks outside areas where the temperature drops below 50-degrees.


Venting

Tanks need to be vented for two reasons.  First, a "normal" or atmospheric vent to allow air into the tank while being filled and emptied.  It also allows equalization of pressure due to normal atmospheric temperature and pressure changes.  Second, an "emergency" vent to help prevent the tank from becoming over-pressurized and rupturing if exposed to fire. For double wall tanks require a second set of normal and emergency vent lines.  Termination of these vents shall be:
  1. Terminated at least 5-feet from any building openings (NFPA 20 section 11.4.1.2.8.2) [Note this is more stringent that the 2-foot requirement of NFPA 37 section 6.7]
  2. Vented so that vapors will not be trapped by eaves or other obstructions  (NFPA 20 section 11.4.1.2.8.2)
  3. [The 2019 edition of NFPA 20 copied over the requirement for the outlets to terminate at least 12 ft above the finished grade]
A code section that is commonly enforced, but not technically applicable is NFPA 30 section 27.8.1 which which applies to Class I liquids.  Diesel fuel is typically a Class II liquid (Flash Point equal to or greater than 100°F, but less than 140°F).  The exact wording is provided for reference below.
27.8.1 Vent Piping for Aboveground Storage Tanks.
27.8.1.1 Where the outlets of vent pipes for tanks storing Class I liquids are adjacent to buildings or public ways, they shall be located so that vapors are released at a safe point outside of buildings and not less than 12 ft (3.6 m) above the adjacent ground level.
You will note that appendix figure A.11.4.4 from NFPA 20 has a diagram indicating a 10-foot minimum distance from the "screened weather vent" to the fill cap.  To the best of our knowledge, there is no specific code requirement for this distance since the appendix is for guidance only.



Sizing of these vents is the per the name-plate on the tanks.  For the normal venting, NFPA 20 (2010 edition) section 11.4.1.2.8 directs you back to ANSI/UL 142 with the additional guidance that the vent shall be at least as large as the fill connection, but not less than 1.25-inch nominal inside diameter.  UL 142 Table 8.2 has the same guidance of a 1.25-inch nominal pipe diameter for tanks under 2,500 gallons in volume.  In practice, most tanks have at least a 2-inch vent connection (2.067 I.D. for Sch-40) as they utilize a 2-inch fill line and cap.

For emergency venting, NFPA 37 is strangely silent. However UL142 section 8.1 states that the tank "... shall have provisions for both normal and emergency venting" and provides the minimum nominal pipe vent diameters based on the wetted surface area in feet.  The key catch is that UL142 is written to only contemplate a maximum nipple length of one foot, so if a longer nipple is attached to the tank (which it always is) it should be calculated.

Fairbanks offers a time saving option of a single combined normal/emergency vent line from the tank (see figure below) which is not prohibited by the codes and standards.  The emergency vent is terminated with a separate weighted emergency vent cap (Item 19), while the normal vent is terminated with a standard screened vent cap (Item 2).  Combining the vents saves labor in the field and penetrations in the owner's walls.

Fill Line

Obviously we need a way to fill the tank.  NFPA 20 does not provide much limitations on the arrangement.  However, we recommend that you check with your local AHJ as one can interpret the code differently. For example, some AHJ's enforce Chapter 34 of the International Fire Code (IFC). Chapter 34 is entitled "Flammable and Combustible Liquids" and if applied to this "above-ground storage tank" would require: An overfill prevention system that would automatically shut off the flow of fuel to the tank (3404.2.9.7.6,1,1.2); Automatic reduced flow rate (3404.2.9.7.6,2); Spill containers having a capacity of not less than 5-gallons at each fill connection (3404.2.9.7.8). 

It is our opinion that NFPA 20 is a more specific code, and therefore the requirements of NFPA 20 override those of the IFC. This is further clarified by IFC section 102.10 which states "Where there is a conflict between a general requirement and a specific requirement, the specific requirement shall be applicable."  This is similar the application of NFPA 30 as noted at beginning of this article.

Remember do not use galvanized or copper pipe for vent or fill connection pipes on diesel tanks.  The sulfur in the diesel fuel can dissolve the zinc in the galvanized plating.  This "sludge" can then clog the fuel pump or injectors over time.

Containment

Per NFPA 20 section 11.4.1.2.4, "Fuel tanks shall be enclosed with a wall, curb, or dike sufficient to hold the entire capacity of the tank".  However, most of the time a double-wall fuel tank with leak detection is acceptable.  This is further clarified by appendix section A.11.4.1.2.1 which states "Dikes are generally not necessary due to the requirement for double-wall tanks with monitoring."  Just don't forget to provide the leak detection for the the interstitial space between the shells of the diesel fuel storage tank.  This signal is to be annunciated by the fire pump controller.

This language also appears to comply with secondary containment requirements of the Environmental Protection Agency (EPA) through the Resource Conservation and Recovery Act (RCRA) contained in title 40 of the Code of Federal Regulations (CFR) part 264.  Diesel fuel is considered a hazardous waste per EPA as they contains heavy metals per Title 40 § 261.3(v) (Definition of hazardous waste).   Per Title 40 CFR 264.175, the worst case of the following conditions shall be contained 1) 150% of the volume of the largest container or 2) 10% of the aggregate volume of all containers.

NEC Classifications of Locations

Do we need to assign any special electrical classification (e.g. Class I, Div 1) due to the presence of diesel fuel?  No.  Generally, diesel has a flash point of 125°F or higher.  As such it is considered a "combustible" rather than flammable per NFPA 30 (Flammable and Combustible Liquids Code). That means it has a flash point above 100°F and, unless you have reason to believe it will be routinely stored or handled at or above its flash-point, it may be ignored as a potential cause for Classifying a location. Note: routinely doesn't necessarily mean commonly or regularly, but simply that it wouldn't be an unusual event. Occasionally, it may be in a blend of fuels that has flammable properties; so it is still a good idea to review the Material Safety Data Sheets (MSDS) for the fuel.

Thursday, April 25, 2013

Fire Pump Sensor Recall

Gem Sensor 3100 Recall Notice

Gem Sensor - Old 3100 model
Gem Sensor - Model 3300 (new)
On April 24, 2012 the Consumer Product Safety Commission (CPSC) issued a press release, CPSC #12‐156, for the voluntary recall of Gems 3100 Pressure Detectors/Transducers because the transducer can fail to accurately detect water pressure in a fire suppression sprinkler system. This could cause the sprinkler system to fail to activate and pump water to the sprinklers in the event of a fire. The failure associated with the Transducer is not a sudden loss of function but rather a slow degradation of performance over many hours of continuous use with constant water pressure. The use of the transducer in non‐water and/or fluctuating pressure applications has not shown the same potential problem.

The sensors are basically failing in a "high" pressure rating (e.g. will read 100 PSI higher than actual pressure).  So it is easy to spot, but does require good on-going maintenance.  One should install a calibrated gauge and compare the pressures.

The transducer has "Gems Sensors & Controls," as well as the 18- digit part number, printed on a label affixed to the center of the transducer. Part numbers beginning with "3100" are included in this recall.

Gems sold the recalled 3100 Pressure Transducers directly to end-users and through distributors from January 2006 through February 2012.

The 3100 has been replaced with the new 3300 series model.  Replacement transducers are avaliable at no cost from Gem.

The newer 3300 model has a green sticker as shown in the figure to the right.  The older 3100 model has a white sticker.



_________________________________________________________________________________

NEWS from CPSC

U.S. Consumer Product Safety Commission

Office of CommunicationsWashington, D.C.


FOR IMMEDIATE RELEASE
April 24, 2012
Release #12-156
Firm's Recall Hotline: (855) 877-9666
CPSC Recall Hotline: (800) 638-2772
CPSC Media Contact: (301) 504-7908

Gems Sensors Recalls Pressure Transducers Used in Fire Pump Controllers Due to Risk of Failure in a Fire

WASHINGTON, D.C. - The U.S. Consumer Product Safety Commission, in cooperation with the firm named below, today announced a voluntary recall of the following consumer product. Consumers should stop using recalled products immediately unless otherwise instructed. It is illegal to resell or attempt to resell a recalled consumer product.

Name of Product: Gems 3100 Pressure Detectors/Transducers

Units: About 25,000

Importer: Gems Sensors Inc., of Plainville, Conn.

Hazard: The transducer can fail to accurately detect water pressure in a fire suppression sprinkler system. This could cause the sprinkler system to fail to activate and pump water to the sprinklers in the event of a fire.

Incidents/Injuries: None.

Description: The Gems 3100 Pressure Transducer is used to detect pressure in a range of applications, including the detection of water pressure as part of a fire pump controller in a fire suppression sprinkler system. The transducer has "Gems Sensors & Controls," as well as the 18- digit part number, printed on a label affixed to the center of the transducer. Part numbers beginning with "3100" are included in this recall.
Sold by: Gems sold the recalled 3100 Pressure Transducers directly to end-users and through distributors from January 2006 through February 2012 for about $250.

Manufactured in: England

Remedy: Contact Gems to receive enhanced twice monthly inspection instructions and information about a free replacement transducer, when warranted. End-users who use the 3100 Pressure Transducer in other applications in which water pressure is measured should contact Gems to determine if their units are affected.

Consumer Contact: For additional information, call the company toll-free at (855) 877-9666, between 8 a.m. and 4:30 p.m. ET, Monday through Friday, or visit the firm's website at http://www.gemssensors.com

Gems 3100 Pressure Transducer

Gem Sensor - Old 3100 model

---
The U.S. Consumer Product Safety Commission (CPSC) is still interested in receiving incident or injury reports that are either directly related to this product recall or involve a different hazard with the same product. Please tell us about your experience with the product on SaferProducts.gov

CPSC is charged with protecting the public from unreasonable risks of injury or death associated with the use of the thousands of consumer products under the agency's jurisdiction. Deaths, injuries, and property damage from consumer product incidents cost the nation more than $900 billion annually. CPSC is committed to protecting consumers and families from products that pose a fire, electrical, chemical, or mechanical hazard. CPSC's work to ensure the safety of consumer products - such as toys, cribs, power tools, cigarette lighters, and household chemicals - contributed to a decline in the rate of deaths and injuries associated with consumer products over the past 30 years.


Under federal law, it is illegal to attempt to sell or resell this or any other recalled product.

Thursday, March 15, 2012

Pressure Relief Valves with Diesel Engines



Photo of Circulation Relief Valve
Circulation Relief Valve
When is a pressure relief valve (PRV) required on a fire pump system?  For a standard 175 PSI rated system, the simple answer is check and make sure that your expected churn pressure plus maximum static suction pressure will not exceed 144.6 PSI (or 206.6 PSI for a 250 PSI rated system).  The sizing of the relief valve comes from NFPA 20 (2010 edition) table 5.25(a) which is summarized in our fire pump sizing app.  For those of you that like to know the details read on.

First let's clarify that we are taking about a main pressure relief valve and not a circulation relief valve.  A circulation relief valve is generally 3/4-inch in size if less than 3,000 gpm and intended to provide a little fresh water into the pump casing for cooling purposes.

photo of main pressure relief valve for fire pumps
Pressure Relief Valve
When we say pressure relief valve in "fire pump" terminology, we are referencing a PRV that is only provided on diesel fire pumps to accommodate possible engine over-speed conditions.  The regulators on diesel engines are good, but not perfect.  When the load on the diesel engine changes,there is a natural delay in response and overshoot while the engine tries to maintain speed.  Our goal is prevent the system from exceeding maximum rated system-pressures when this slight change in RPM occurs.  By code diesel fire pumps have a kill switch should the RPM exceed ten-percent of the design speed.  In practice, we have rarely seen a well maintained engine exceed 1.5% of the required speed when adjusting for loads.

So how does this 10% maximum permitted over-speed RPM relate to the output pressure?  The speed (RPM) of a fire pump and the pressure developed do not increase linearly.  That is if you double the RPM you do not double the pressure.  When you double the speed you actually get four times the pressure.  The formula is written as follows:

Pump Speed vs. Pressure Affinity Formula
Pump Speed vs. Pressure Affinity Formula - Rearranged

If we square the 10% increase in speed, we get a 21% increase in pressure as shown below.  If we take 144.6 psi * 121%, we get 175 PSI.  In practice, we do not recommend running your system this close to the rated system pressure unless you know there will never be any change to your suction pressure (i.e. a vertical turbine fire pump).  For diesel fire pumps we recommend that you call and talk to one of our sales engineers.
The NFPA 20 fire pump committee did this exact same exercise and came up with the following language for the code:
NFPA 20 – 2010 Edition
4.18.1* General.
4.18.1.1 Where a diesel engine fire pump is installed and where a total of 121 percent of the net rated shutoff (churn) pressure plus the maximum static suction pressure, adjusted for elevation, exceeds the pressure for which the system components are rated, a pressure relief valve shall be installed.

FM Global Standard 3-7 (Fire Protection Pumps – May 2010)
2.3.3 Pressure Relief Valves

2.3.3.4 Provided a main relief valve on all diesel engine-driven fire pumps when 121% of the net rated shutoff (churn) pressure plus the maximum pump static suction pressure exceeds any system component rated pressure.
FYI - Remember that utilizing a Pressure Relief Valve to reduce discharge pressures is not permitted by NFPA 20 section 4.7.7.2. Designing system to discharge large amounts of water every time it runs is not considered good design practice and code recognizes this fact.