Process pressure transmitter with seal having diamond like carbon coating

A process pressure transmitter system includes a process pressure transmitter housing, a process pressure sensor in the process pressure transmitter housing, a flange face in the process pressure transmitter housing and an isolation diaphragm on the flange face. A first capillary passageway carries a first fill fluid from the isolation diaphragm to the process pressure sensor. A process seal diaphragm couples to a process fluid of the industrial process. A second capillary passageway carries a second fill fluid from the process seal diaphragm to the isolation diaphragm. A diamond like carbon (DLC) coating coats the process seal diaphragm.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority of Chinese patent application Serial No. 201410306576.0, filed Jun. 30, 2014, the content of which is hereby incorporated by reference in its entirety.

BACKGROUND

The present invention relates to the process control industry. More specifically, the present invention relates to an isolation diaphragm or seal of the type used to couple a process control instrument to a process.

Some types of process control instruments, such as pressure transmitters, have a pressure sensor which is fluidically coupled to an isolation diaphragm by a fill fluid. The isolation diaphragm comprises part of a subassembly called a “remote seal” or a “diaphragm seal” and isolates the pressure sensor from corrosive process fluids being sensed. Pressure is transferred from the isolation diaphragm to the sensor through the fill fluid which is substantially incompressible and fills cavities on both sides and a capillary tube (or thru-hole if the seal is directly mounted to the instrument). The tube is typically flexible and may extend for several meters. The process medium contacts the remote isolation diaphragm which conveys the exerted pressure to the pressure sensor disposed in the transmitter housing.

Typically, the isolation diaphragm and any process wetted parts of the remote seal are made of a corrosion resistant material such that the process medium does not damage the diaphragm. It is also known in the art to provide a coating on the isolation diaphragm in order to protect the isolation diaphragm from corrosion due to contact with the process fluid. However, there is an ongoing need for improved isolation diaphragm protection.

SUMMARY

A process pressure transmitter system includes a process pressure transmitter housing, a process pressure sensor in the process pressure transmitter housing, a flange face in the process pressure transmitter housing and an isolation diaphragm on the flange face. A first capillary passageway carries a first fill fluid from the isolation diaphragm to the process pressure sensor. A process seal diaphragm couples to a process fluid of the industrial process. A second capillary passageway carries a second fill fluid from the process seal diaphragm to the isolation diaphragm. A diamond like carbon (DLC) coating coats the process seal diaphragm.

This Summary and the Abstract are provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. The Summary and the Abstract are not intended to identify key features or essential features of the claimed subject matter, nor are they intended to be used as an aid in determining the scope of the claimed subject matter.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention is a diaphragm seal for coupling a process variable transmitter to an industrial process fluid. The diaphragm seal includes an isolation diaphragm which is coated with diamond like carbon (DLC) to protect the isolation diaphragm from damage due to contact with industrial process fluid.FIG. 1shows a remote seal12of a process variable transmitter11Remote seal12is connected to a transmitter diaphragm in housing14. Remote seal12includes a housing17and is configured to couple to process fluid16.

Pursuant to one embodiment, transmitter11measures the pressure of process medium16. Remote seal12includes a thin flexible diaphragm18which contacts process medium16. Seal12also includes backplate19which, together with diaphragm18, define cavity20. Capillary tube22couples cavity20to pressure sensor28disposed in transmitter housing14, such coupling being made via transmitter housing diaphragm25and a sealed fluid system connecting diaphragm25with sensor28. The sealed fluid system, as well as cavity20and capillary tube22, is filled with a suitable fluid for transmitting the process pressure to sensor28. The fluid may include silicone, oil, glycerin and water, propylene glycol and water, or any other suitable fluid which preferably is substantially incompressible.

When process pressure is applied from process medium16, diaphragm18displaces fluid, thereby transmitting the measured pressure from remote seal12through a passage in plate19and through tube22to pressure sensor28. The resulting pressure applied to pressure sensor28, which can be a capacitance-based pressure cell, causes such capacitance to change as a function of the pressure at medium16. Sensor28can also operate on other known sensing principles, such as strain gauge technology. Circuitry within transmitter housing14electronically converts the capacitance into a linear 4-20 mA transmitter output signal over wire pair30related to the process pressure. Any appropriate communication protocol may be used including the HART® communication protocol in which digital information is modulated on to a 4-20 mA current, the Foundation Fieldbus or Profibus communication protocols, etc. Process control loop30may also be implemented using wireless communication techniques. One example of wireless communication technique is the WirelessHART® communication protocol in accordance with IEC 62591.

FIG. 2Ais a side cross-sectional view,FIG. 2Bis a bottom plan view andFIG. 2Cis a top plan view of a remote seal50. Remote seal50is referred to as a, “flanged-flush design” and includes seal housing52. Remote seal50also includes a hydraulic fluid (fill fluid) fill port54, an instrument connection56, and a flexible diaphragm58which is welded by TIG weld60. Surface62provided which is an annular shape and extends around diaphragm58. Bolt holes64are used for coupling housing52to, for example, a tank filled with process fluid.

Typically, housing52is formed from stainless steel and has a thickness of about 1 inch. Housing52is machined in a manner to be welded to the circular metal diaphragm58. Gasket surface62is also machined on housing52. Diaphragm58is typically a foil diaphragm that may be cut and formed with a die press.

FIG. 3is a simplified block diagram showing pressure transmitter system10in which process pressure sensor28is positioned in process pressure transmitter housing14. As illustrated inFIG. 3, isolation diaphragm25is carried on a flange face80of housing14. A first capillary passageway82carries an isolation fill fluid and extends from diaphragm25to the pressure sensor28. Process diaphragm seal18couples to a process fluid and a second capillary passageway22carries a second fill fluid and extends from the process seal diaphragm18to the isolation diaphragm25. As a pressure is applied to diaphragm18, the diaphragm18flexes. This causes the pressure to be transferred through the second fill fluid to isolation diaphragm25. In turn, isolation diaphragm25flexes and causes the pressure to transferred to the fill fluid in capillary passageway82. This can be sensed by pressure sensor28in accordance with known techniques. Transmitter electronics88are used to sense the applied pressure and communicate the information related to the applied pressure to another location.

FIG. 4A,FIG. 4BandFIG. 4Care side cross-sectional views showing example configurations of seal12. InFIGS. 4A and 4B, a diamond like carbon (DLC) coating90is applied to an outer surface of the diaphragm18. In the configuration ofFIG. 4A, the DLC coating90only covers the diaphragm18. This configuration will protect the diaphragm18from abrasion. Such a configuration may reduce costs and simplify manufacturing. Further, in this configuration, a weld92which is used to weld the diaphragm18to a flange face94of the remote seal12may be performed after the coating procedure occurs. This simplifies the manufacturing process.

FIG. 4Bshows another example configuration in which the DLC coating90extends across the entire wetted surface including face94. This provides corrosion resistance to all of the surfaces of the seal12which may contact process fluid.

FIG. 4Cis another example configuration in which the DLC coating90is provided on an interior surface of diaphragm18. This configuration is useful to prevent hydrogen permeation through the diaphragm18. In such a configuration, the coating90is applied to diaphragm18prior to welding diaphragm18to flange face94.

The DLC coating90can be deposited using any appropriate technique. For example, the coating can be deposited using a physical vapor deposition technique. For example, a filtered cathodic vacuum arc (FCVA) deposition apparatus can be used to deposit the DLC coating90. In such a device, an electrical arc is applied to a cathodic material causing vaporization and ionization of the material. A magnetic filter field is used to filter the vapor and a magnetic focusing field is used to focus the resulting carbon plasma onto the surface of diaphragm18in a vacuum chamber. Various types of diamond like carbon may be deposited as desired.

FIG. 5is an enlarged cross-sectional view of a portion of diaphragm18. As illustrated inFIG. 5, an intermediary layer100is deposited between diaphragm18and DLC layer90. The intermediary layer can be selected to promote adhesion of the DLC layer90to the diaphragm18. The intermediary layer100can be configured to improve the surface roughness of the diaphragm18as well as provide a transition layer having a thermal expansion coefficient which is between the thermal expansion coefficient of diaphragm18and DLC coating90. Further, the intermediary layer100can protect the material of diaphragm18during the deposition process and improve the corrosion resistance of the final structure. For example, the DLC layer90may develop pin holes. Similarly, pin holes may be developed in the diaphragm18during the deposition process. The intermediary layer100protects the diaphragm18during this process. The intermediary layer100may be selected as desired. In one configuration, the layer100comprises titanium. However, other example materials including tantalum, chromium, or ceramic materials.

Different types of diamond like carbon coatings may be used to fabricate layer90. In one example embodiment, DLC layer90comprises the diamond like carbon coating comprises a-C:H diamond like carbon. In another preferred embodiment, the coating comprises ta-C DLC.

Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention. The remote seal may be of a configuration other than those specifically illustrated herein. Examples include flanged seal types such as a flushed flange seal, an extended flanged seal or a pancake seal. Other configurations include threaded seals (RTW), union connection seals, chemical tee seals, threaded pipe mount seals, saddle and flow-through seals, etc. The capillary passageway22may be elongate such as that illustrated inFIG. 1, or, in another example configuration, may be relatively short whereby the transmitter mounts directly to the seal. In one configuration, the diaphragm18comprises stainless steel. Other example materials include Hastelloy®, tantalum, titanium, Nickel 200/201, Alloy 400 (Monel™), Alloy 625 and 600 (Inconel™), super duplex stainless steel 2507, zirconium, gold, silver, platinum, nickel based alloys, refractory metals and noble metals. In one configuration, any welding is performed prior to depositing the DLC coating. This ensures that the welding process does not damage the DLC coating. The thickness of the DLC coating may be chosen as desired. For example, the thickness may be between 0.5 and 5 μm. In some configurations, a thinner layer is preferred so long as it meets application requirements. Although a single intermediary layer is illustrated, any number of intermediary layers may be employed as desired.