8 | Baker Hughes
Copyright 2020 Baker Hughes Company. All rights reserved.
Scenario 2: 2:1 Turndown Application
This quantity is measured by flow rate, and the heat is measured by enthalpy, H.
From the law of conservation of mass, we can substitute outlet with the combination of inlet + water.
This equation can then be rearranged to solve for the water mass flow rate, water.
The water source used for steam conditioning should
be more than 150 PSI (11.5 Bar) above the outlet steam
pressure. This range ensures proper atomization and
a good range of performance. A higher water pressure
differential is also desirable. Water pressure below 150
PSI is also acceptable; however, low-pressure differential
limits the amount of turndown the system is able to
provide. The minimum operating differential pressure is
typically between 20 PSI and 45 PSI, depending on the
nozzle design that is being used.
Each spray nozzle also has a maximum rated pressure
differential. However, this is not a concern because the
excess pressure differential is distributed across the
water control valve.
Water Temperature
The temperature of the water source is a variable that
directly impacts the amount of water required to reduce
the steam to a specified outlet temperature. The water
temperature also has a direct effect on the amount
of downstream pipe that is required and dictates the
efficiency of evaporation.
Cold water sources, such as condensate and makeup
water, require less mass flow to sufficiently cool the
steam to the outlet set point. The exact amount of water
required can be determined through a heat balance
calculation. This calculation is a balance of the quantity
of heat entering the valve in the forms of inlet steam and
spray water, to the quantity of heat exiting the valve as
the outlet steam.
Hot spray water, such as boiler feed water, requires a
larger quantity of flow, but evaporates at a faster rate
because less heat transfer is required to reach the
saturation point. High temperature water also reduces
thermal shock by minimizing the differential temperature
between the steam and water.
For the majority of the cases, hot water is beneficial to
steam conditioning as it minimizes the downstream pipe
requirements and helps protect against water fallout.
However, in low-pressure steam applications, hot water
can sometimes result in flashing as the pressure is
reduced through the control valve or nozzle orifice. If the
flashing occurs upstream of the nozzle, special valve trim
is required to prolong the active life of the equipment. If
the flashing occurs upon injection into the outlet pipe, it
will be beneficial in assisting with evaporation and mixing
of the water with the steam. The water temperature
is a design condition that typically cannot be modified
for a particular application. However, it is important to
understand the impact the water source has and design
the system to suit the condition.
x
x
x
x
x
x
x
