Showing posts with label Diesel Engines. Show all posts
Showing posts with label Diesel Engines. Show all posts

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, 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.