Ambient Temperature

Ambient Temperature: is the temperature of the surroundings

When a Diaphragm Seal System is situated outdoors, the ambient temperature can become a very important factor for the functionality of the Diaphragm Seal System. Ambient temperatures can range from extremely high (>60°C e.g. in desert sun) to low (<-40°C, e.g. offshore wind). Ambient temperatures should be specified with a minimum and maximum value, e.g. -10/+35°C would be a general specification for the Netherlands.

Design temperature

Design Temperature: both the lowest and highest temperature that can occur in a given process specification

The design temperature is reached when the process runs out of control. Design temperatures need to be taken into consideration during design phase to ensure mechanical integrity of the device when exposed to these design temperatures. Proper functioning after this exposure is not required

Operating temperature

Operating temperature:
Minimum: the lowest temperature under which the process still runs stable
Normal: the temperature under which the process runs optimally
Maximum: the highest temperature under which the process still runs stable

Operating temperatures need to be considered to ensure proper functioning of the device and as such they are important to select the correct Diaphragm Seal design and fill fluid. The device will operate effectively within a specified operating temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the maximum operating temperature (or peak operating temperature). Outside of this range, the device may fail. Each Diaphragm Seal has limitations according its construction or base material. For the flange connection a guide is provided, showing the maximum and minimum temperature ratings. This guide also details limitations on the flanges concerning at what pressure they can be used at specific temperatures. The maximum operating temperature is limited by the sealed system component with the lowest maximum temperature. The limiting component may be, but is not limited to, any one of the following: housing material, diaphragm material, gasket or o-ring material, or fill fluid.

Temperature Effects

Changes in the volume of the Diaphragm Seal System can be caused by changes in volume and density of the fluid in the system and occur when the fill fluid expands or contracts due to fluctuations of the process and/or ambient temperatures. This change in fill volume drives a change in the internal pressure of the Diaphragm Seal System. This is called the temperature effect.

There are 3 different influences that can be distinguished:

Process temperature: the process temperature has effect on the Diaphragm Seal system and influences, in combination with the lowest process pressure, the selection of the fill fluid.

Ambient temperature: Differences in ambient temperature have an effect on the viscosity and on the density of the fill fluid. The change in viscosity has a direct effect on the response time of the Diaphragm Seal System. The change in density creates a mounting effect in case of a vertical height difference which results in a zero-point deviation of the Diaphragm Seal System.

Delta T in DP measurement: Temperature effects can cause a difference in density or viscosity between the HP side and LP side of the Diaphragm Seal System.

Minimising Temperature Effects

Temperature effects can be minimised by using the following techniques:

Reduce the volume of the application: This can be done with reducing the capillary internal diameter (ID). However, reducing the capillary ID increases the response time for the application and in this a balance needs to be found. Badotherm standardly uses 2mm as inside diameter of the capillary. Switching to capillary with ID of 1 mm will increase the response time with a factor 16. Capillary with 1mm inside diameter is advised for gauge pressure applications with a range above 100 barg. It is also important to minimise the volume of fill fluid contained in the pressure instrument to a minimum. This can be done by replacing the standard process covers of a transmitter by special designed low volume covers, or to use pressure gauge with a reduced volume.

Specify the shortest possible capillary lengths for the application: it is understandable that for interchangeable reasons the capillary lengths for all LT tags in a project are specified with the same length. It is advised that capillary lengths should be calculated on a tag by tag basis to define the shortest possible length for each tag. Too long capillaries will result in unacceptable response times and/or too high temperature influences with all consequences for the reliability of the measurement.

Cold temperature application: there are processes where, due to a system of both low ambient and process temperatures, the contraction of fill fluid is as such that the diaphragm runs against the diaphragm chamber and transfer of pressure is no longer possible. A solution for these situations is a so-called ‘supplementary fill’ which allows for pressure transfer even when the fill fluid heavily contracts.

Insulation of Diaphragm Seal System: in cases where both capillaries and Diaphragm Seals are exposed to severe outdoor conditions, insulating them will reduce ambient temperature effects considerably. Also in those cases where the capillary of one leg of a DP system is in the sun and the other leg in the shadow insulating them will result in the same or similar ambient temperature at both legs and minimises ambient temperature effects.

Tracing of capillaries: this will reduce ambient temperature effects close to zero. Often tracing is considered expensive and requires intensive maintenance. However, a new development is that Badotherm is working together with O’Brien to include the product Tracepak around capillary lines already at the time of assembly of the Diaphragm Seal system.

High temperature process and temperature reducers: in those processes with high temperatures (> 200°C) and where distant mounting by means of capillary is not possible, a temperature reducer (TR) can be foreseen between instrument and Diaphragm Seal. A temperature reducer reduces process temperatures to a value < 100°C at the instrument side.

Temperature compensator: when the specifications of the process prescribe the use of a fill fluid with high viscosity this might result in unacceptable response times. A temperature compensator (TC) is developed to overcome this problem. By using the high viscous fill fluid in the part between seal diaphragm and TC, the fluid keeps a relative high temperature resulting in a low viscosity; after the TC a standard fill fluid with low viscosity is used resulting in an acceptable overall response time.

Process simulation: Badotherm offers the possibility of a full process simulation. The Diaphragm Seal System is simultaneously subjected to: process temperature, ambient temperature, and static process pressure. Their respective influences on the calibrated (DP) span are recorded. This results in a fingerprint for this specific Diaphragm Seal System and the influences can be incorporated into the DCS to compensate for the effects. The result is a pressure (level) measurement with enhanced accuracy.

LGP solution: is designed for Low Gauge Pressure measurement and eliminates temperature and mounting influences. Test results obtained by measuring low gauge pressures with a standard Diaphragm Seal and with the LGP, showed that the minimum range reduces from 80 mbar for a standard seal to only 10 mbar with the LGP. Similarly the ambient temperature effect reduces from 0.4 mbar per 10°C to 0.03 mbar, while the process temperature effect reduces from 0.39 to 0.09 mbar.