The remote seal also referred to as diaphragm seal are connected to a measuring instrument using a direct connection or a capillary.

A remote seal system consists of an external sensing diaphragm mounted to the process and connected to a differential pressure transmitter with oil-filled capillaries. The capillaries and seals are filled with incompressible oil suited with process pressure and temperature. The transmitter may be mounted at a certain distance or nearby the high-pressure process connection.

For transmitters under vacuum are mounted below the high-pressure side tap to reduce the effects on the transmitter fill fluid.

Advantages of the remote seal capillary transmitters:

• Plugging caused by viscous processes and suspended solids with piped impulse lines can be overcome with capillary seals.

• Capillary systems are most flexible and not rigid like piped impulse lines.

• As Capillary system makes it possible to easily install around permanent structures, confined spaces.

• Transmitters can be mounted at a certain safe distance from the process pipelines or tanks. 

1. Remote capillary Transmitter mounted at zero levels.
2. Remote capillary transmitter mounted below the seals
3. Remote capillary Transmitter mounted above the top seals.
4. Remote capillary transmitter between the seals.
5. Remote capillary transmitter mounted below the seals on the Closed Tank.
6. Remote capillary transmitter mounted on the Closed Tank.

1.Remote capillary Transmitter mounted at zero levels.

Hd = Vertical distance from the transmitter to high-pressure seal

Ld = Vertical distance from the transmitter to low-pressure seal

Lmax = The maximum process level above the high-pressure seal.

Lmin = The minimum process level above the high-pressure seal.

Sf = Specific gravity of remote fill fluid

Sp = Specific gravity of process fluid

Tank span = Lmax *Sp – Lmin * Sp 

          = (100 in.* 0.55) – 0 in*0.55) = 55 inH2O.

4 mA = Lmin *Sp + Hd *Sf

= (0 in.* 0.55) + (0 in.* 01.09) = 0 inH2O

20 mA = Lmax * Sp + Hd * Sf

 = (100 in.* 0.55) + (0 in.* 01.09) = 55 inH2O 

 Span = 55 inH2O (55 to 0 inH2O)

Note: Both of the installations would have the same calculated range points.

Note: Silicone 704 has a specific gravity of 01.09.

2.Remote capillary transmitter mounted below the seals.

Tank span = Lmax* Sp – Lmin * Sp

= (100 in.* 0.55) – (0in *.55 in) = 55 In H2o

4 mA = Lmin *Sp + (Hd* Sf)

= (0 in.* 0.55) + (60 in.* 01.09) = 65.4 inH2O

20 mA = Lmax *Sp + (Hd *Sf)

= (100 in* 0.55) + (65.4) = 120.4 inH2O

Span = 55 inH2O (120.4 to 65.4 inH2O)

Note: Silicone 704 has a specific gravity of 01.09.

3 . Remote capillary Transmitter mounted above the top seal

Tank span= Lmax Sp – Lmin* Sp

= (100 in. 0.55) = 55 inH2O

4 mA = Lmin * Sp +(Hd * Sf)

 = (0 in * 0.55) + (-120 in * 01.09) = –130.8 inH2O

20 mA = Lmax * Sp + (Hd * Sf)

= (100 in * 0.55) + (-120 in.* 01.09) = –75.8 inH2O

 Span = 55 inH2O (–130.8 to –75.8 inH2O)

4. Remote capillary transmitter between the seals.

Tank span = Lmax SG

= 100 in. 0.55 = 55 inH2O

 4 mA = Lmin *Sp – (Ld* Sf) +(Hd * Sf)

= (0 in. 0.55) – (60 in.* 01.09) + (– 60 in.* 01.09) = –130.8 inH2O

 20 mA = Lmax * Sp – (Ld * Sf) +(Hd Sf)

= (100 in. 0.55) – (60 in. 01.09) + (–60 in. 01.09) = –75.8 inH2O

Span = 55 inH2O (–130.8 to –75.8 inH2O)

Note: Silicone 704 has a specific gravity of 01.09.

5. Remote capillary transmitter mounted below the seals on the Closed Tank

Tank span= Lmax *SG 

= (100 in.* 0.55) = 55 inH2O

4 mA = Lmin * Sp – (Ld * Sf)+ (Hd+Sf)

= (0 in.* 0.55) – (120 in.* 01.09) + (60in * 1.09) = – 130.8 inH2O

20 mA = Lmax *Sp – (Ld* Sf) + (Hd+Sf)

= (100 in.* 0.55) – (120 in.* 01.09) + (60in * 1.09) = –75.8 inH2O

Span = 55 inH2O (–130.8 to –75.8 inH2O)

Note: Silicone 704 has a specific gravity of 01.09.

• Remote capillary for closed Tank

Tank span= Lmax *Sp – Lmin* Sp

= (100 in.* 0.55) = 55 inH2O

4 mA = Lmin * Sp – (Ld * Sf)

= (0 in.* 0.55) – (120 in.* 01.09) = – 130.8 inH2O

20 mA = Lmax *Sp – (Ld* Sf) 

= (100 in.* 0.55) – (120 in.* 01.09) + (60in * 1.09) = –75.8 inH2O

Span  = 55 inH2O (–130.8 to –75.8 inH2O)

Note: Silicone 704 has a specific gravity of 01.09.

Author: PSS Bapu Rao

Source: Emerson