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Article # 0030

Comparison of Automatic Recirculation Valves With Conventional Bypass Recirculation Systems in Boiler Feedwater Applications

 

By Richard L. Jones, PE

 

Minimum flow recirculation systems for protection of centrifugal pumps in boiler feedwater applications have long been utilized.  The primary purpose of the bypass on a centrifugal pump is to prevent excessive temperature rise in the pump at very low flow rates.  A bypass has thus been especially necessary for boiler feedwater pumps.  The various types of bypasses utilized have included continuous flow, on/off, and modulating flow control.  

 

A continuous flow bypass system requires that the pump be oversized to continuously pass the minimum recommended flow amount through a bypass line downstream of the pump to the deaerator regardless of the process demand. As the pressure in the fluid will be reduced from the elevated pressure at the pump outlet to the near atmospheric pressure of the deaerator, a fixed orifice would be installed in the bypass line as close to the deaerator as possible to provide back-pressure and cavitation and flashing protection to the upstream piping.  However, this back-pressure protection exists only at the higher flow rates as the intermediate pressure drops as the flow decreases.  This type of bypass system is highly reliable, has a lower installation cost, and requires almost no maintenance.

 

 Continuous flow bypass recirculation systems are now rarely utilized due to the increased cost required to oversize the pump and motor and to the additional energy usage necessary to operate the over-sized pump even when the process demand exceeds the minimum flow requirement.  The increased energy costs to continuously bypass are significant.  For example:

 

Assume a Boiler Feed Pump flowing 500 gpm of water, having 600 ft. of head at the 200 gpm minimum flow, and an efficiency of 70%.  Further assume that this pump operates 24 hours per day for 335 days per year and the plantís cost of electricity is $0.05 / kw-hr.

 

The equation to calculate the brake horsepower to drive a pump:

 

bhp = (Flow) (Ht) (S) / (3,960) (e)

where:

Flow is pump output in gpm

Ht is Head in ft.

S is specific gravity of the liquid

e is pump efficiency in per cent

 

Total pump = (500gpm) (600 ft.) (1) / (3,960) (0.70) = 108 bhp

Pump bypass = (101 bhp) (200gpm/ 500gpm) = 43.3 bhp

 

Using the conversion factor 0.7457 kw/bhp:

Pump bypass = (43.3 bhp) (0.7457 kw/bhp) = 32.3 kw

 

The pump operates:

(335 days/yr) (24 hr/day) = 8,040 hr/yr

 

Thus, in one year, the bypass would require:

(32.3kw) (8,040 hr) =259,692 kw-hr of electricity

 

At the plantís cost of $0.05/kw-hr, the continuous bypass would cost the plant annually:

(259,692 kw-hr) ($0.05/kw-hr) = $12,695 annual energy cost due to continuous bypass

 

Controlled bypass systems include both on-off and modulating.  In the on-off bypass system, an automated valve is opened and closed to completely bypass the required minimum pump flow.  Although the energy loss is significantly less than in the case of continuous bypassing, this type is not energy efficient as it will always bypass 100% whenever the process demand falls even slightly below the minimum required.   For this reason, most current boiler feedwater bypass systems are of the modulating type.

 

A current conventional modulating type bypass system consists of the following key components:

1) A check valve in the main pump discharge line to prevent reverse flow.

2) A recirculation control valve with integral cavitation trim located in the bypass line.

3) A flow sensor/meter at the pump suction.

4) Wiring between the DCS, the meter, and the control valve.

5) Air supply to the control valve including gages, filters, and regulators.

6) A check valve in the bypass line.

 

The benefits of the conventional modulating type bypass system include energy efficiency as the flow being bypassed is limited to the specified minimum.   

 

Another type of modulating bypass control is an Automatic Recirculation Valve (ARV). An ARV encompasses all of the hardware required in the conventional bypass system in a single housing.  The ARV has an integral main-line check valve which not only protects against reverse flow but provides excellent pulsation dampening of the fluid.  The recirculation control valve located in the ARV bypass is designed to handle the entire pressure drop from the main boiler feedwater pressure to the deaerator without any additional pressure letdown devices. The bypass valve is  also a non-return valve protecting against reverse flow from the deaireator.  The ARV operates by sensing the main flow and positioning the bypass control accordingly, thereby avoiding electrical power and control wiring and the need for instrument air supply. 

  

The advantages of an ARV as compared to a conventional system include the following:

1) A single valve replaces numerous components resulting in a significant savings in installed cost.  It also provides single source responsibility for the performance of the recirculation system.

2) In contrast to the complex combination of equipment and power requirements of a conventional bypass, the ARV is a simple and proven design which provides long-term reliability with little or no maintenance.

3) Dampening of the impeller generated low-frequency pulsations protects the pump and overall system.

4) No loss in the energy savings as it is modulating control.

5) As an integral extension of the ARV, a variable orifice back pressure can be provided to maintain a constant back-pressure to the bypass outlet of the ARV regardless of flow rate.  Such a device is often recommended to avoid flashing and the potential damage to the piping downstream of the automatic recirculation valve. 

 

A properly designed and maintained system for protection of a boiler feedwater pump is essential to controlling plant costs and minimizing downtime.  An automatic recirculation control valve provides the energy efficiency, low installed cost, and long term reliability necessary to provide this critical function in a modern power generating facility.

 

Richard L. Jones is the President of Richard L. Jones, Inc. and a 1975 Nuclear Engineering graduate of Texas A&M University


Article # 0030         TEST QUESTIONS:

1.   The primary purpose of the bypass on a centrifugal pump is to prevent excessive temperature rise in the pump _______.

  1. at very high flow rates

  2. at very low flow rates

  3. at normal operating conditions

  4. All of the above

2.   What are the two main types of bypasses discussed in this paper?

  1. Continuous and Controlled

  2. Continuous and Automatic recirculation valves

  3. Controlled and Modulating

  4. Mechanical and Electronic

3.   A continuous flow bypass system requires that the pump ____.

  1. be oversized

  2. run continuously

  3. has a 2 inch outlet

  4. All of the above

4.   Two disadvantages to Continuous flow bypass recirculation systems are _____. 

  1. increased equipment and energy costs

  2. the increased diameter and amount of piping

  3. increased installation and maintenance costs

  4. decreased reliability and lower flow rates

5.   Which of the following is a type of controlled bypass system?  

  1. continuous

  2. modulating

  3. on-off

  4. b. and c.

6.   In which of the discussed bypass systems is an automated valve opened and closed to completely bypass the required minimum pump flow.

  1. on-off

  2. continuous

  3. modulated

  4. none of the above

7.   A current conventional modulating type bypass system typically does NOT include the following component?

  1. A flow sensor/meter at the pump suction.

  2. A check valve in the bypass line.

  3. A pressure gauge in the bypass line

  4. A recirculation control valve with integral cavitation trim located in the bypass line.

8.   What is an ARV?

  1. Automatic Rate Variation

  2. Automatic Recirculation Valve

  3. Advance/Retardation Valve

  4. None of the above

9.   An advantage of an ARV as compared to a conventional system is ...

  1. a single valve replaces numerous components.

  2. the ARV provides long-term reliability with little or no maintenance.

  3. it does not need electrical power, control wiring or an instrument air supply

  4. All of the above

10.   The recirculation control valve located in the ARV bypass is designed to handle the entire pressure drop from the main boiler feedwater pressure to the deaerator without ______.

  1. additional pressure letdown devices

  2. requiring a manual control valve

  3. any temperature drop in the bypass line

  4. none of the above

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