Best practices for industrial boiler feedwater systems

Boiler insights

• In a boiler system, the deaerator is essential to its successful operation.
• Valves can help control the pressure within the deaerator, along with controlling the steam used as the heat source.

Boiler feedwater is the single most important system in the boiler house, and at its heart is the deaerator. The deaerator combines treated makeup water and condensate and using low-pressure steam, removes dissolved oxygen and other noncondensable gases by raising the fluid to the saturation temperature.

No revelations here; however, the method by which returned condensate is delivered can greatly affect the desired steady state system operation.

We normally encounter high-pressure (HP) condensate from the traps on the steam lines and pumped condensate returned from the users throughout the steam distribution system. The high-pressure condensate gets delivered directly to the steam space in the storage section where it flashes into steam and liquid at the lower operating pressure.

Pumped condensate has already cooled below saturation temperature and has been exposed to air in vented receivers so it must pass through the deaerating section with the makeup water (see Figure 1). Condensate is normally collected in a surge tank in the boiler house from the distribution system, then pumped into the deaerator, but needs to be delivered in a controlled manner, otherwise the deaerator will experience pressure fluctuations and flashing of the stored liquid. Figure 1 will highlight these effects.

Figure 1: Typical industrial deaerator flow diagram. Courtesy: CDM Smith

Boiler system calculations

Consider a 150 pounds per square inch gauge (psig) system, designed to deliver 100,000 pounds per hour (PPH) of saturated steam, with a distribution system that returns 50% of its steam as condensate and a deaerator operating at 10 psig. The deaerator mass and energy balance yield a steam flow rate of ~11,100 PPH¹ given those conditions.

This calculated flow would be considered the average design rate; however, as one of my mentors would say, when you place one foot in a bucket of ice water and the other in a bucket of boiling water the “average” is fine but you will lose both feet. To understand how the system will operate, we need to look at the maximum and minimum events of an on/off cycle of a “pumped down” condensate delivery system.

Performing the mass and energy balance with no pumped condensate flowing, the steam required at the deaerator is ~16,900 PPH¹ and then only ~6,200 PPH² when pumping down the surge tank, but now consider this happens suddenly when the float actuates the pump and the corresponding reactions.

The system is operating, the condensate pump is off and then the float activates the pump. We are suddenly presented with too much steam flow resulting in a pressure spike in the deaerator until the steam control reacts to the excessive change. A minute or so later, we get the sudden pressure drop, when the condensate flow is abruptly stopped and the makeup water flow increases again (3,100 back to 16,900 PPH) requiring a rapid increase in steam flow to maintain pressure. This oscillation will occur continually with a frequency depending on the load, surge tank and pump sizes.

The initial pressure spike at reduced steam loads will be less intense then at full load, however still significant for deaerator operation. In this half load case the steam flow will oscillate from ~8,530 to ~5,600 PPH³ between the condensate pump being off and then turned on. This is then followed by a second pressure spike when the pump turns off due to the increased storage volume and no immediate need for makeup water flow. Very little steam is required until the storage level falls sufficiently to require the makeup water valve to again begin flowing. Note that the condensate pump delivers the same mass to the deaerator regardless of current steam demand requirements.

Deaerator pressure swings

The continual pressure swings depicted can have a long-lasting negative effect on the entire boiler system. The pressure swings can cause system piping to shake and liquid to flash in the storage section leading to support issues and cavitation in boiler feedwater pumps. Additionally, the pilot operated self-regulating type steam pressure reducing valves usually employed will experience premature diaphragm failures under these conditions and are unable to respond effectively with these type of pressure swings.

The following remedy (Figure 2) employs the additions of a control valve, restriction orifice (RO), level transmitter and indicator into an existing condensate system. Instead of on/off control a pump runs continuously and the control valve operates proportionally between closed at the pumps NPSHR and 100% open at just below the overflow.

Figure 2: Surge tank modifications. Courtesy: CDM Smith

This operation will allow for a continual condensate flow to the deaerator, roughly equal to what is being returned and thereby maintain the deaerator’s desired steady state conditions. The pump is protected against dead head flow by the RO and requires no operator interaction other than manually switching the operating pump to distribute operating hours and maximize pump life.

System calculations need to be performed at the minimum load conditions to properly define the criteria for the steam pressure reducing and makeup water flow control valves. Don’t be surprised if multiple steam valves are required to properly cover summer to winter conditions when the steam is also being used as a heating source. Having encountered this scenario multiple times, the solution presented is a cost-effective retrofit with minimal impact to operations when being installed and commissioned.

Figure 3: Spray type deaerator, makeup water level control. Courtesy: CDM Smith

Mass balance¹:

STM + CR(HP) + CR(P) + MU = BFW

STM + 25 + 50,000 + MU = 102,000

MU = 51,975 – STM

Heat balance¹:

STM * h_g + CR(HP) * h_L + CR(P) * h_L + MU * h_L = BFW * h_L

STM * (1196.2) + (25 * 327.8) + (50,000 * 148) + (51,975 – STM) * 13 = 102,000 * 207.9

STM * (1196.2) + (51,975 – STM) * 13 = 13,797,605

STM = 11,090

MU = 40,885

Legend:

STM – 10 psig steam reduced from the 150 psig saturated steam header

CR(HP) – High-pressure condensate returns directly from the 150 psig saturated steam header

CR(P) – Pumped condensate returned via atmospheric vented condensate receiver(s)

MU – Makeup water

BFW – Boiler feedwater at 10 psig saturated

h_g&L – Specific enthalpy (British thermal unit/#)

Note: When pumped condensate exceeds BFW requirements, MU is zero and the BFW mass equation is substituted into the heat balance equation to solve for STM.

1. Assumptions: 2% boiler continuous blowdown (CBD), 25 PPH HP condensate, 180°F pumped condensate, 45°F makeup water and no heat loss.

2. Same conditions as above except pumped condensate flow is nearly doubled (~93,000 PPH) to pump down the receiver.

3. Same conditions as above except 50,000 PPH steam demand and 3% CBD.

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