ST 79 – How often should I test steam quality?

Steam Quality Testing – Frequency

AAMI/ANSI ST79 (2017) “Comprehensive guide to steam sterilization and sterility assurance in health care facilities”

How often are facilities supposed to test steam quality?

Here is what the Standard says:

  • upon installation or relocation of the sterilizer
  • after any change to the steam distribution lines or boiler supply water
  • when investigating sterilization process failures
  • when periodically assessing sterilization loads for wet packsSteam quality testing frequency

Steam Trap Testing

Testing steam traps using temperature involves measuring the temperature of the steam entering the trap. Here are the steps to do so:

1. Identify the steam trap you want to test. It could be a specific trap or a series of traps in a steam system. We recommend all the traps between the boiler and sterile processing be identified and set up on a 6 week schedule.

2. Locate the steam trap’s inlet connection. The inlet is where the steam enters the trap, and includes the inlet strainer.

3. Use a non-contact infrared thermometer to measure the temperature of the steam entering the trap at the inlet connection.

Infrared thermometers work by detecting the infrared radiation emitted by an object and converting it into a temperature reading. They use a sensor called a thermopile, which consists of multiple small thermocouples connected in series. When the sensor is exposed to infrared radiation, each thermocouple generates a small electric voltage proportional to the temperature difference between the object and the sensor.

The thermopile converts the voltage into a temperature reading, which is displayed on the thermometer. Some infrared thermometers also have a laser pointer that helps to target the object being measured.

It’s important to note that infrared thermometers measure the surface temperature of an object and not the internal temperature.However, shiny surfaces, such as polished metal or glass, have a high reflectivity, which means they reflect a significant amount of infrared radiation rather than absorbing it. This reflection can interfere with the accuracy of the temperature reading obtained by the infrared thermometer. As a result, the thermometer may measure the temperature of the reflected surroundings rather than the actual temperature of the shiny surface itself.

On stainless steel or other shiny piping, spraying a small spot of black high temperature paint increases the infrared sensing accuracy. On 100 psi steam we are looking for 300F, on 60-70 psi sterilizer steam we are looking for 270F. Any less means the trap may be plugging.

An ultrasonic stethoscope is a device used to detect and analyze ultrasonic sound waves. When it comes to testing steam traps, here’s how an ultrasonic stethoscope typically works:

1. Ultrasonic Detection: Steam traps are designed to remove condensate from steam systems. Over time, steam traps can develop faults or fail, leading to energy losses and decreased system efficiency. An ultrasonic stethoscope detects high-frequency sound waves produced by steam leaks or the presence of condensate in faulty steam traps.

2. Probe Placement: The ultrasonic stethoscope is equipped with a probe or sensor that is placed near the steam trap being tested. The probe is designed to pick up ultrasonic signals emitted by the steam trap.

3. Sound Analysis: The ultrasonic stethoscope amplifies and converts the detected ultrasonic sound waves into audible signals that can be heard by the user. These signals are then analyzed to determine the condition of the steam trap.

4. Interpretation: By listening to the sound signals, experienced technicians can identify specific patterns or anomalies associated with faulty steam traps. These patterns might include hissing, gurgling, or turbulent sounds caused by steam leaks or blockages.

5. Maintenance and Repair: Once a faulty steam trap is identified, appropriate maintenance or repair actions can be taken to rectify the issue. This might involve cleaning, replacing, or adjusting the steam trap to restore its proper functioning.

It’s worth noting that the specific operation of an ultrasonic stethoscope can vary depending on the manufacturer and model. Therefore, it’s always recommended to consult the user manual or receive proper training to ensure accurate and effective use of the device for testing steam traps.

 

Steam Quality Moore1

Simple Steam Quality Tests

Want to do some simple tests to check your steam quality?

  1. Weigh a textile pack before the cycle, run your standard cycle but no dry Weigh it again. The % weight gain should be around 1%.

(Put the dry time back to its original setting.)

  1. Tape a non-woven wrapper to the Load Car shelves, full length and Load the Car. Again, run a cycle with no dry time.

After unloading, look at the amount of water on the wrappers.

The “scientific terms” employed here for moisture are BEADS, PUDDLES and LAKES.

For good quality steam, there should be only a few “beads” observed under the heavy part of the shelf structure.

The top shelf should be dry. (When done, put the dry time back to its original setting.)

  1. If you want to see the amount of moisture created by a tray of metal items, leave the non-woven wrap in place on one of the lower shelves, place the loaded tray on the shelve above without any wrapper or other absorbent material on/in the

Run the cycle with no dry time and see how much moisture is on the wrapper below. (Again, put the dry time back to its original setting.)

Quantifying Steam Quality

One equation comprising two independent variables gives an accurate assessment of steam quality

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Reprinted from Plant Services

Steam quality is a measure of the amount of saturated steam that coexists with its condensate in a given system. Calculate it by dividing the mass of steam by the total mass of steam and condensate.

Steam quality = Msteam/(Msteam + Mcond) (Eqn. 1)

Others have pointed out the importance of identifying steam quality in systems supporting industrial applications. For example, excessive moisture in the form of free droplets carried along with the main steam flow might impinge on turbine blades and cause mechanical damage. Likewise, high-velocity condensate can score valve seats and cause other erosionand corrosionrelated problems.

Ganapathy(1) gives a detailed explanation of how to calculate steam quality with a throttling calorimeter. A small quantity of the steam is throttled through an orifice from system pressure (PS) down to atmospheric pressure. The steam temperature at the orifice exit (TE) is recorded. This expansion is adiabatic. The following expression describes the energy balance associated with the throttling process: HM = HL (1-X) + HGX (Eqn. 2)

And after rearranging:

X = (HL-HM)/(HL – HG) (Eqn. 2a)

Where:

HM = enthalpy of superheated steam at temperature.

TE = exit temperature at atmospheric pressure.

HL and HG = the enthalpy of condensate and steam, respectively, at system pressure PS.

X = the steam quality.

Thermodynamic data for calculating steam quality may be obtained from ASME Steam Tables(2) or any other library source(3). Ganapathy developed a diagram, which displays TE on the abscissa and X on the ordinate. A series of isobars for PS in the 50 to 500 psia range also is shown on the diagram. The diagram provides a quick estimate of steam quality when TE and PS are known.

Liley(4) calculated steam quality at pressures ranging from 2 bars (29 psia) to 20 bars (290 psia) in one bar increments. He developed an equation of the form:

X = A + BTE (Eqn. 3)

It describes the steam quality at each pressure, PS as a function of TE. Each set of coefficients A and B is valid for only a single pressure.

Coefficients for pressures not included in the list must be interpolated. Solving equations 2a and 3 requires a user to look up recorded data.

The following equation was developed to quantify steam quality when the pressure and calorimeter temperature are known. It is valid for a steam quality between 0.95 and 1.00 and for pressures between 30 psia and 600 psia:

X = 0.9959 – 0.000442TE – ln[(PS + 6.8)0.03218(PS + 374)-0.0001581TE] (Eqn. 4)

Solving for TE:

TE = [0.9959 -X – 0.03218 ln(PS + 6.8)]/[0.000442 – 0.001581 ln(PS + 374)] (Eqn. 4a)

Expressing steam quality by means of a single continuous function eliminates the need for graphical data representation or interpolation. The equations can be used for online steam quality monitoring with a programmable process controller using orifice exit temperature and steam system pressure as input values, or they can simply be stored in the memory of a pocket calculator for use when the information is required.

By curve fitting available data in the ASME Steam Tables, the P-T relationship for saturated steam in the 30 to 600 psia range can be expressed as:

PS = 1.5 + (TS/120.62)4.5886 (Eqn. 5)

Solving for the saturated steam temperature TS:

TS = 120.62 (PS – 1.5)0.21793 (Eqn. 5a)

Substituting Eqn. 5 into Eqn. 4 allows the latter to be expressed entirely in terms of TE and TS. The graph in Figure 1 gives a quick method for calculating steam quality when saturation pressure and temperature and calorimeter temperature are known.

The validity of the above equations has been tested against a known example presented in one of the referenced articles and, at a more elevated pressure level, against steam data obtained from the Engineering Data Book(3).

Figure 1. Steam quality X can be quickly assessed graphically when the exit temperature TE and the saturated steam temperature TS and pressure PS are known.

Try these examples

Example 1. Calculate the steam quality of a 200 psia system when the orifice exit temperature is 250 F.

Liley, using the method described in his article, calculates the steam quality to be 0.9636. Applying Eqn. 4 with TE = 250 and PS = 200 gives an answer of 0.9649a difference of 0.13 percent.

Example 2 uses thermodynamic data obtained from the Engineering Data Book. It calculates steam quality at a pressure higher than that described in either of the two referenced articles:

Example 2. Calculate the quality of steam at 566.1 psia, when the throttling calorimeter temperature is 300 F.

The calculations use available data from the GPSA Engineering Data Book. At 566.1 psia, the enthalpy of the condensate is 464.5 Btu/lbm while the enthalpy of the saturated steam is 1204.1 Btu/lbm. The enthalpy of expanded steam at 300 F and 14.696 psia is 1,192.8 Btu/lbm. Substituting into Eqn. 2 gives: 1,192.8 = (1-X)(464.5) + 1,204.1 X. Solving for X yields 0.9840. Using Eqn. 4 instead results in an answer of 0.9836a difference of 0.004 percent.

Example 3. Steam at 500 psia is to have a quality of not less than 0.9775. What should be the expanded steam temperature?

Eqn. 4a provides the answer. The calculated temperature is 289.5 F. This means the steam temperature at the orifice exit must be no less than 290 F.

Example 4. A saturated steam system has a temperature of 460 F. Expansion in a throttling calorimeter results in a steam temperature of 300

  1. Calculate the steam quality.

Start with Eqn. 5 to calculate system pressure when the temperature is 460

  1. This gives a calculated pressure of 466.58 psia. This answer differs by

0.06 percent with the value noted in the Steam Tables. The calculated pressure and expanded steam temperature are then entered in Eqn. 4. The steam quality is 0.9844.

Example 5. Calculate steam quality when saturated steam at 420 psia shows a temperature difference of 165 F between steam temperature and calorimeter temperature.

Use Eqn. 5a to calculate the saturated steam temperature: TS = 449.50 F. Then TE = 449.5 – 165 = 284.50 F. Now substitute PS = 420 and TE =

284.5 into Eqn. 4. Answer: X = 0.9756. Also refer to Figure 1 for comparison.

References:

  • Ganapathy, , “Calculate the moisture content of steam,” Chemical Engineering, p. 127, August, 1993.
  • Meyer, A., McClintock, R.B., Silvestri, G.J., and Spencer, R.C., Jr.,

ASME Steam Tables, 5th edition, A.S.M.E., New York, 1983.

  • Engineering Data Book, Section 24, “Thermodynamic Properties,” Revised 10th edition, Gas Processors Association, Tulsa, Oklahoma,
  • Liley, Peter , “A simple equation for steam quality,” Chemical Engineering, p. 140, August, 1994.

Frank A. Rusche is a process engineer with Kellogg-Brown and Root in Houston, Texas. He can be reached at Frank.Rusche@Halliburton.com or 281-575-3432.

 

ELLISON-STYLE STEAM THROTTLING CALORIMETER

Exclusive U-Path Principle

Pioneered by Ellison in the early 1900’s to provide accurate readings for pressures down to 26 PSIG. The steam flows through the inlet holes of the sampling nozzle, through the valve assembly and the orifice plug, expanding into the steam chamber. The steam flows into the bottom of the inner chamber, flashing moisture, then up the sides to the top where it encapsulates inner chamber before exhausting at the bottom of calorimeter. This process causes inner chamber to have stable thermal properties.

Throttling – Why?

Throttling of steam provides accurate steam quality results over a wide range of process conditions. If superheat is maintained within Ellison calorimeter, exacting calculations can be performed.

High Moisture Content

Small amounts of moisture separated in the inner chamber are re-evaporated when unit returns to superheat temperature range. A drain valve connected to inner chamber is used to collect separated liquid during high moisture periods.

Insulation

The steam chamber is designed so escaping steam flows around inner chamber to ensure a uniform temperature within the calorimeter. The steam jacket is packed with one inch of very low thermal conductivity insulating material to give additional thermal protection.

NOTE: Cal Research’s study has found the indicated steam quality is that of the steam sample entering the calorimeters and if the sample is not iso- kinetic to the steam within the line, errors will occur. Herein lays the importance of a well designed steam sampling probe assembly. This means if the sample is from the side or bottom of the line, the steam sample will be wet.

Accuracy

The Ellison Steam Calorimeter has a general accuracy of the +/- ½% total reading when operating temperature of unit indicates superheat. This can be enhanced, with corrections for sample flow, orifice flow, line thermal losses, and isokinetic condition, to +/- .1% total reading.

 

Lowspots

Steam Distribution Piping: Low spots

The steam distribution piping must be pitched properly to avoid low spots. The steam piping loses heat by radiation, causing condensate to form. This liquid water accumulates on the inside surface of the pipe wall. During periods of low flow this water runs by gravity and will accumulate in any low spot. When the flow increases, velocity causes waves toform at the surface of the accumulated condensate. When one of the waves touches the top of the pipe, a slug is formed, which
is driven at high velocity downstream by the steam pressure differential. Because the steam velocity can accelerate these slugs to 60 mph or greater, drip traps, and even separators have difficulty catching all the condensate and draining it from the steam main.

Steam traps are needed at low spots or risers in a steam system to remove condensate (liquid water) that forms during the process of steam generation and distribution.

When steam is generated, it contains a certain amount of water vapor. As the steam travels through pipes and equipment, it cools down and loses heat energy. This causes the water vapor to condense back into liquid form, known as condensate.

If condensate is allowed to accumulate in low spots or risers, it can cause several problems. Firstly, condensate takes up space in the pipes, reducing the effective flow area for steam and hindering the efficiency of the system. Secondly, condensate can cause water hammer, a phenomenon where the rapid movement of condensate can create pressure surges that can damage equipment and pipes. Lastly, condensate may contain impurities that can corrode the system or affect the performance of downstream equipment.

Steam traps are designed to automatically remove condensate from the steam system while preventing the loss of steam. They work by opening to allow condensate to drain out and closing to prevent the escape of steam. By installing steam traps at low spots or risers, condensate can be effectively removed, maintaining the efficiency and integrity of the steam system.