Guide to Inspections of Low Acid Canned Food 16
Manufacturers - 2
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curve produced by the temperature recorder. Modulating systems control the steam valve to proportionally supply steam to the system as needed.
One method of retort temperature control found prior to the current FDA lacf regulations in some U.S. lacf plants, and still found in lacf plants in foreign countries, is the operator manually controlled steam valve. In these retort systems the operator observes the steam pressure on a retort pressure gauge and/or the retort temperature on a MIG thermometer, or on some other type of thermometer. The steam valve is then opened and closed by the retort operator to maintain the required steam pressure or thermometer readings. In some plants where this type of control has been used, it has been noted that by assigning one operator to each retort, the steam supply to the retort can be controlled to provide for a consistent temperature. The retort temperature control is dependent upon the operator observing the retort steam pressure and/or temperature constantly. This type of steam/temperature control does not meet the requirements of §113.40.
The temperature of saturated steam of high quality bears a fixed relationship to its absolute pressure (psia). In most instance steam is measured as gauge pressure (psig), which has its zero point at 14.7 psia (equivalent to atmospheric pressure at sea level). 15.1 psig (Pounds per Square Inch Gauge Pressure) of steam at sea level is equal to 250° F. Attachment 1 is a table which provides the corresponding gauge pressure and process temperatures at various altitudes. Gauge pressures above those listed for the corresponding temperatures may indicate air pockets in a steam retort system.
One of the oldest methods of temperature control in steam retort systems is the use of the relationship between steam pressure and temperature in the retort. In this type of control system a pressure line is connected between the retort and a steam control valve. The diaphragm on the control valve is opposed by a spring which pushes against the valve stem and opens the valve. When the steam is turned on to the retort the steam pressure in the retort is low and the steam control valve is forced open by the spring tension. As the pressure/temperature in the retort rises, steam pressure against the diaphragm forces the steam valve to close, and less steam is supplied to the retort. As the pressure/temperature in the retort fluctuates the valves opens and closes to modulate the steam supply to the retort. This type of controller may provide for adequate temperature control if it is well maintained. You will normally see fluctuations in the temperature recorder curve as the pressures changes in the retort and steam supply. Firms using this type of controller normally operate several degrees above their filed scheduled process temperature to account for fluctuations in the temperature in the retort.
Control of steam supplies to retorts has been reported using valves which are connected to the steam header. One type of valve is actuated in one direction by a lever arm with adjustable weights, operating against an opposing fixed spring. In another type the valve is set to open a fixed amount by adjusting the compression of a moveable spring, operating against an opposed fixed spring. If the steam header pressure does not fluctuate appreciably, these will perform satisfactorily. If the header pressure does vary, the retort pressure/temperature will vary also; for these valves are essentially steam pressure ratio valves.
For example: Assume that the header pressure is 105 psig when a valve of this type is adjusted to hold 15 psig (250° F) in the retort. If the header pressure drops to 90 psig at any time, the retort pressure will drop to 12.5 psig (245° F).
(P1-P2)/P2 = pressure ratio
therefore (105-15)/15 = 6
and (90-15)/X = 6, or X = (90-15)/6 = 12.5 psig
For this reason in those systems which use these types of controllers it is important to observe the retorts operate through several cycles to determine if temperature fluctuations are being caused by fluctuations in the steam supply. Using this type of steam controller to supply steam to more than one retort would not meet the requirements of 21 CFR 113.40. Fluctuations in temperature control should be documented during the inspection. Records should be reviewed to determine if process deviations have occurred because of fluctuations in steam supply to the retorts. The use of equipment which does not meet the intent of the regulations is a FDA 483 item.
Self-actuated steam supply/temperature control valves which operate directly off of the pressure generated in a liquid/vapor filled temperature sensing tube are available for temperature control in industrial situations. In this type of control system the vapor generated by the heating of the liquid in the temperature sensing bulb is used to exert pressure on the diaphragm of the steam control valve. The pressure on the diaphragm is opposed by a spring. As the liquid heats up in the sensing tube the gas pushes on the valve diaphragm causing the valve to close. As the temperature drops the spring forces the valve open. Different liquids are used to
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