Firewater pumps

In the third e.vample, the line terminates at 53%. This means DO NOT run this pump at less than 53% of the BEP. 53% of 4500 gpm is 2385 gpm. Because this is a firewater pump and because firemen need to throttle the nozzles on their fire hoses, then we need to install a pressure relief valve on this system with a discharge bypass line so that the pump dumps the restricted water (less than 2400 gpm) back into the suction tank or lake. If not, this firewater pump is likely to suffer bearing failure during an emergency. 

The next graph is a typical family curve for a firewater pump (Figure 7-14) ... 

Prime movers are typically fueled by natural gas or diesel. Dual fuel turbine units exist that can run on natural gas and can automatically switch to diesel. So-called dual fuel reciprocating engines run on a mixture of diesel and natural gas. When natural gas is not available, they can automatically switch to 100% diesel. Most prime movers associated with producing facilities are typically natural gas fueled due to the ready availability of fuel. Diesel fueled machines are typically used to provide stand-by power or power for intermittent or emergency users such as cranes, stand-by generators, firewater pumps, etc. 

Several catastrophic fire incidents in the petroleum industry have been the result of the facility firewater pumps being directly affected by the initial effects of the incident. The cause of these impacts has been mainly due to the siting of the fire pumps in vulnerable locations without adequate protection measures from the probable incident and the unavailability or provision of other backup water sources. A single point failure analysis of firewater distribution systems is an effective analysis that can be performed to identify where design deficiencies may exist. For all high risk locations, fire water supplies should be available from several remotely located sources that are totally independent of each and utility systems which are required for support. 

The metallurgy selected for construction of a firewater pump is dependent on the properties of the water source to be used. For fresh water sources (i.e., public water mains), cast iron is normally adequate although bronze internals may be optional. Brackish or sea water utilization will require the use of highly corrosion resistance materials and possibly coatings. Typically specified metals include alloy bronze, monnel, ni-resistant, or duplex stainless steels sometime combined with a corrosion resistant paint or specialized coating. 

Small capacity pumps commonly referred to as "jockey" pumps are provided on a firewater system to compensate for small leakages and incidental usage without the main pump(s) startup. They are set to start 0.70 to 1.05 kg/sq. cm. (10 to 15 psi) above the start up pressure of main firewater pumps. In some cases a cross-over from the utility water system can be used in place of a jockey pump, however a check valve is installed to prevent drain down of the firewater by the utility water system. Jockey pumps do not require the... 

Firewater storage tank, firewater pump, emergency power - diesel, electric. 

If instmments are placed on manual, particularly emergency instruments, then a common cause event has been created. For example, one of the contributing factors in the Piper Alpha disaster was that both firewater pumps were on manual. They had been placed in that state to ensure that any divers in the water were not sucked into the firewater intake in the event of an automatic start of the pumps. Unfortunately, the fire on the platform prevented operators firom getting to the pumps to turn them on. The lack of firewater capability materially contributed to the magnitude of the disaster. 

Maintenance located a 12 in. 300 psi RF check valve that was used on firewater pumps, but not on hydrocarbons. This substitute valve was manufactured to a Japanese flange to flange standard dimensioa The Japanese standard face to face dimensions are more compact than the ANSI standard so the storeroom stocked this valve with a connecting short spool piece of 12 in. piping (see Fig. 2.27 to imderstand the dimension). 

There should be at least two firewater pumps located at different places on the platform and with redundant drivers (usually diesel motors). 

Firewater pumps, pumping capacity and pressures, activation ... 

To avoid common failure incidents, prime mover and backup fire pumps preferably should not be located immediately next to each other and ideally should be housed at separate locations at the facility. They should feed into the firewater distribution at points that are as remote as practical from each other. In practical applications, except for offshore installations, most small to medium sized facilities contain a single firewater storage tank, requiring the siting of all firewater pumps close to it. Even in these circumstances it may be wise to segregate the main and backup fire pumps from each other with tie-in points to the firewater distribution loops. This mostly depends on the hazard level of the facility and... 

For onshore facilities, water may be supplied from local public water mains, storage tanks, lakes, and rivers. In these cases, a conventional horizontal pump is typically employed. The preferred design for onshore firewater pumps is a horizontal centrifugal type with a relatively flat performance curve (i.e., pressure versus volume).The discharge pressure is determined by the minimum residual pressure required at the most remote location of the facility, flowing its highest practical demand with allowances added for piping friction losses. 

When more than one pump is installed, they should be coordinated to start in sequence, since immediate startup of all pumps may not be necessary and could cause damage to the system. Depending on the number of pumps available they can be set up on sequentially decreasing fire main pressure set points. All firewater pumps should be able to be started from remote activation switches located in manned control rooms, but shutdown should only be accomplished at the pump itself. 

Firewater pumps should be solely dedicated to fire protection. They may be used to feed into a backup system for emergency process cooling but not as the primary supply. If such backup is allowed, it should be tightly controlled and easily accessible for prompt shutdown in case of a real emergency. 

A method of testing firewater pumps should be provided to verify adequate performance. Additionally, most fire protection audits, insurance surveys, and local maintenance requirements require firewater pumps to be routinely tested for performance verification. In fact, predictive maintenance can be performed before the firewater pumps reach reduced flow or pressure performance levels requiring removal. Pressure gauges on the suction and discharge should be provided and methods to verify the flowing water quantity at each test point. The sizing of the flow in test piping should account for the maximum flowrate of the unit, not just its rated capacity. 

The purchase or specification of a firewater pump to support process facility operations should be in compliance with recognized international standards for such equipment. The most commonly referred to standards are listed below. All of these standards require a factory acceptance test of the unit. 

In some cases, the proximity of a local fire station or provision of a dedicated fire station within a large industrial complex can be relied upon to provide backup firewater pumping capabiHty to the fire protection system. In fact, historical evidence indicates that when the fixed firewater pumps have been impacted by a major fire or explosion incident, mobile fire apparatus has to be heavily relied upon as a backup mechanism. Previous coordination with the fire station as to their capabilities, mobile apparatus accessibility, connection points, drafting sites, emergency admittance, and manpower should be evaluated and incorporated into emergency pre-fire plans for the facility. 

Process facilities susceptible to earthquakes should be provided with suitable restraints for fire protection systems. The extent of these restraints are normally dictated by local ordinances and primarily concern the bracing of pipework and adequate securing of firewater pump base plates and controller panels for earthquake forces. Pump houses should be adequately constructed and braced so they will not collapse onto the firewater pump or distribution piping. 

Start the fire pump to be tested and let it run for a minimum of 30 min, for stabilization of the mechanical systems. The firewater pump can be started manually from the controller, but it is typically preferred that a local fire water device is opened (i.e., fire hydrant(s)) to simulate firewater pump auto-start on low fire main pressure detection. If several fire pumps are arranged to start in sequence, the sequence startup should be verified to confirm programming logic arrangements. Adjust the driver (i.e., engine) rpm to operate the pump as close as possible to the rated rpm performance curve. 

Plot the test points against the rated pump curve adjusted for the rated rpm of the firewater pump under testing. If conditions permit, data should be plotted immediately during the test to indicate abnormalities that may be corrected, e.g., partially closed or open valves. 

Verify firewater pump startup on low pressure indication if such capability has been provided and tested while flow performance testing is undertaken. 

Verification of fire pump startup and flow and pressure indications in the plant DCS should be confirmed if such indications are provided as part of the plant monitoring system. Remote stopping of the firewater pump should not be allowed. 

Even though CO2 will not bum, its presence can affect the flammability range and characteristics of the hydrocarbons with which it is mixed. (It will generally reduce both the lower and upper flammability limits.) Also, the high concentrations of CO2 in air that are likely to be present during a major fire may affect the performance characteristics of the diesel motors used to drive the firewater pumps. 

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