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BFW Systems - BFW Pumps
February 27, 2005

Introduction
BFW Pump Capacity
BFW Pump Suction
BFW Pump Discharge
Drivers
Minimum Recycles
Standbys


Introduction

Boiler feed water pumps supply heated deaerated water to the waste heat boiler for the production of steam on a continuous and reliable basis.

Boiler feed water pumps are generally multistage horizontal centrifugal pumps.  In cases where the flow is small, positive displacement reciprocating pumps have been used.

BFW Pump Capacity

The BFW pump capacity is established by adding to the maximum boiler flow a margin to cover boiler swings and eventual reduction in capacity due to wear.  In other industries (steam power plants) this margin can range from 20% for a small plant to 5% for large plants.  For acid plants a margin of 20% is generally specified.

BFW Pump Suction

Careful attention should be paid to the sizing and design of the suction piping.  The aim is to minimize pressure drop in the piping to ensure that the NPSH available (NPSHA) is always greater than the NPSH required (NPSHR) under all possible operating conditions.  This becomes even more important when the fluid being pumped is near or at its boiling point as in a boiler feed water pump.

NPSH is calculated as follows:

                    NPSHA = Ha + Hh - Hf - Hvp

                    where    Ha   - Absolute pressure above surface of liquid
                                 Hh     - Static head, from liquid surface to pump centre-line
                                 Hf    - Friction loss in suction piping
                                 Hvp   - Vapour pressure of liquid at the pumping temperature

In the case of a BFW pump taking water from a deaerator, the value of Ha and Hvp are equal and cancel each other out.  We are left with the NPSHA equal to the difference in the static head and piping friction losses.  If piping losses are minimized, the elevation of the deaerator is minimized.

The static head (Hh) is measured from the surface of the liquid to the pump centre-line.  The lowest expected liquid level should be used rather than the normal operating level.   This will usually correpsond to the low/low alarm point in the deaerator.

The worst operating case should be assumed for the piping friction losses.  Use the maximum expected flowrate for this calculation.

As a safety margin, ensure that the NPSHA is at least 20% higher than the NPSHR by the pump under the worst operating conditions.

The suction piping should be as short and direct as possible with the minimum number of bends.  One pump vendor suggests that the suction piping be at least one size larger than the pump suction nozzle.

During commissioning the suction piping is usually flushed to remove pipe scale and debris from construction.  A temporary strainer is usually installed in the suction line during initial startup and left installed until it is assurred that the line is clean.  The strainer is usually a 'witch's hat' and the free area of the mesh should contain at least 3 times the area of the suction pipe.

BFW Pump Discharge

The discharge piping should contain an isolation valve and check valve.  The check valve should be installed between the pump discharge and the isolation valve.  The purpose of the check valve is to protect the pump from excessive pressures and prevent reverse flow through the pump.  The pump discharge should also be equipped with a minimum recycle.

Drivers

The two most common drivers for BFW pumps are electric motors and steam turbines.   Electric motors are generally used on the normally operating pump while the steam turbine is used on the stanby pump.  The steam turbine will generally be a single-stage turbine with the exhaust steam discharged to a low pressure header.

Minimum Recycles

At low or no flow conditions, a multistage high-head BFW pump can easily generate excessive pressure and heat which can lead to pump failure.  To prevent this, a 'leak-off' system must be provided to ensure that a minimum flow is maintained through the pump.  The pump vendor generally specifies the minimum flow requirements for their pumps.  Some pumps may require a mimimun flow of at least 25% of the rate pump flow.

This requirement can be achieved in a number of ways but in general consists of a recycle line back to the deaerator with orifices and valves to control the amount of flow.

The simplest system consists of a recycle line with orifice(s).  The orifices are sized for the minimum flow rate as specified by the vendor and to let down the pressure from the pump discharge.  This system maintains a continuous recycle flow even during normal operation.  The BFW pump capacity must be specified allowing for this continuous recycle flow.

An alternative is to use a special valve known as a recirculation valve.  As the flow decreases, the valve automatically opens the recycle line maintaining the minimum required flow at all times.  As flow to the boiler increases, the recirculation valve automatically closes the recycle.  The recirculation valve provides some pressure reduction in the recycle flow, but the system should be checked for to determine if additional orifices are required for pressure reduction.

A third method uses instrumentation to detect flow and open a control valve in the recycle line.  A flow meter in the main feedline measures the water flow to the boiler.   When the flow drops below the minimum flow specified by the pump vendor, a controller begins opening the recycle control valve in the recycle line.  When the flowmeter measures zero flow the control valve will be fully open.  Orifices should be installed in the recycle line if additional pressure reduction is required.

Standbys

Generally, a standby pump is specified as backup to the normally operating pump(s).  The standby pump should start automatically if the pressure drops in the feed water line or the operating pump fails.

At least one of the pumps must be capable of operating during a power failure.  This is accomplished by having at least one pump driven by a steam turbine.  Even if there was a site power failure there would be enough high pressure steam remaining in the steam drum and headers to operate a turbine drived BFW pump until the steam system could be safely shutdown.  A motor driven pump on emergency power could also be used.

For steam trubine driven BFW pumps on standby, the turbine must be kept warm by operating the unit on a slow roll.  A small amount of steam is bled to the steam turbine to keep the casing and impeller warm and slowly turn the turbine and pump.  The pump vendor must be informed of this type of service to ensure that the pump bearings are specified correctly.