Sizing an Orifice Meter Blowdown Flow Control
August 19, 2014 by Aaron Rhoade
Overview
There are many different manufacturers of devices used to control the continuous blow down from an industrial boiler from simple needle valves to sophisticated electronic systems. The Madden Orifice Meter is unique in design. It has been in production for over 60 years and has proved to be a simple, reliable blow down flow control for continuous boiler blow down. How do you select the correct model for your boiler installation?…
Here are our recommendations on sizing an Orifice Meter.
- Background: There are two types of blow down from a large industrial boiler. The bottom blow down is done intermittently and requires the opening of two valves, one a quick opening and the other slow. This intermittent bottom blow down is done for a short period, often just seconds, and it is done to clear the sediment and gunk out of the bottom of the boiler. The continuous blow down comes from the top, from the steam drum of a water tube boiler, or somewhere in the upper half of a fire tube boiler. Continuous blow down is used for controlling water and steam quality, and that plays a vital role in maximizing operating efficiency. As its name says, continuous blow down creates a continuous flow of hot water from the boiler. The purpose of the continuous blow down operation is to help the boiler operator maintain the target level of total dissolved solids (tds) in the boiler water.
- Step 1: First we need to know the operating pressure of the boiler. Madden offers two models: the model no. OM250 is rated for use on boilers up to 250 psi; and the model no. OM650 is rated for boilers up to 650 psi. So we use the boiler operating pressure to select the Orifice Meter model with a maximum rating that equals or exceeds the boiler operating pressure. Step 2: The next step is to select the orifice plate to be installed in the Orifice Meter, because the orifice plate we select will determine the range of flow that we can get from the unit.
- To select an orifice plate we need to know the estimated blow down flow range from the highest to the lowest rate per hour as measured in US pounds. The boiler operator or the water quality technician may need to be consulted to get this figure. If continuous blow down flow figures are not available then we may still be able to do an estimate. For example, if the boiler is rated for up to 800 boiler horse power, or 27,600 lbs/hr (800 bhp X 34.5 lbs/hp) then we can figure that the maximum blow down will not be more than 10% of the boiler capacity, or 2,760 lbs/hr. Many large water tube boilers are operated with a target maximum blow down rate of 3% of the boiler capacity, so in that case, a 100,000 lbs/hr boiler capacity would be 3,000 lbs/hr of blow down.
- Reminder: Many boilers have higher operating rates in the winter than they do in the summer, so you will want to make sure that any seasonal operating differences are included in your calculations for the maximum and minimum continuous blow down volume requirements.
- Example: So now let’s take an example to look at: an 800 HP boiler operating at 150 psi, with an expected maximum blow down rate of 2,500 lbs/hr, and a minimum rate of 600 lbs per hour. If we consult the Orifice Meter Flow Chart found in the data sheet form no. OM09 (or Click here to see the on line flow charts) we will be able to select to orifice plate that will give us the range we need. Starting with the Model OM250 section for the 1A Orifice Plate we see columns starting at 50 psi and going to 250 psi along the top of the chart. Using the 150 psi column, because 150 psi is our boiler operating pressure, we can see that with the Orifice Plate no. 1A installed our minimum flow capacity at the orifice no. 1 would be 130 lbs/hr, and our maximum setting using orifice no. 17 would be 2,115 lbs per hour. Since that is not enough for our maximum estimated continuous blow down of 2,500 lbs/hr we can reject the Orifice Plate no. 1A and move on to Orifice Plate no. 2A.
- Example Conclusion: Using the 150 psi column for the Model OM250 section for the 2A Orifice Plate, we can see that with the Orifice Plate no. 2A installed our minimum flow capacity at the orifice no. 1 would be 501 lbs/hr, and our maximum setting using orifice no. 17 would be 3,900 lbs per hour. The 2A orifice plate will cover our maximum and minimum requirements. So our product selection is OM2502A.
Reliable blow down flow control is essential for optimizing boiler and steam system performance and efficiency. The Madden Orifice Meter blow down flow control is a proven device used in plants all over the world.
Madden technical representatives are available to help select the right model for your application.
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