Pressurized dual packing seal valve

1. A valve system comprising:

a valve, the valve comprising:

a valve seat;

a valve plug configured to control a flow of process fluid through the valve according to a separation between the valve plug and the valve seat;

a valve stem in mechanical communication with the valve plug and configured such that linear actuation of the valve stem controls the separation between the valve plug and the valve seat;

first and second packing seals surrounding the valve stem, each of the packing seals forming a seal between the valve stem and a surrounding gland housing, the first and second packing seals being separated from each other along the valve stem by a seal gap, a spacer being provided within the seal gap, the first and second packing seals being adjacent to opposing ends of the spacer; and

a seal pressurization port configured to allow a seal pressurizing fluid to enter into the seal gap and to be pressurized within the seal gap to a desired gap pressure;

a pressurization fluid source configured to supply the pressurizing fluid;

a pressurization fluid line through which the pressurizing fluid flows from the pressurization fluid source to the seal pressurization port of the valve,

a pressurizing fluid pressure regulating apparatus configured to regulate a pressure of the pressurizing fluid in the pressurization fluid line; and

a flow measuring device configured to measure a flow rate of the pressurizing fluid from the pressurizing fluid pressure regulating apparatus to the seal gap, said flow of the pressurizing fluid being unrestricted between the pressurizing fluid pressure regulating apparatus and the seal pressurization port of the valve;

wherein the seal pressurizing fluid that leaks past the first packing seal flows between the valve stem and the gland housing and is released directly into the environment, thereby requiring an increased flow rate of the pressurizing fluid into the seal gap so as to maintain the desired gap pressure; and

wherein the seal pressurizing fluid that leaks past the second packing seal flows between the valve stem and the gland housing and is released directly into the process fluid, thereby requiring an increased flow rate of the pressurizing fluid into the seal gap so as to maintain the desired gap pressure.

2. The valve system of claim 1 , further comprising a pressure measuring device configured to measure a pressure of the pressurizing fluid within the seal gap.

3. A method of preventing process fluid leakage along a valve stem of a linear stroke valve, the method comprising:

providing a valve system according to claim 1 ;

determining or estimating a process pressure of the process fluid;

applying the pressurizing fluid to the seal pressurization port, the pressurizing fluid being pressurized to a gap pressure that is higher than the process pressure;

monitoring the flow rate of the pressurizing fluid from the pressurizing fluid pressure regulating apparatus to the seal gap; and

applying a maintenance action to the valve after the monitored flow rate changes by more than a specified amount.

4. The method of claim 3 , further comprising monitoring a pressure of the pressurizing fluid in the pressurization fluid line.

5. The method of claim 4 , further comprising applying the maintenance action to the valve after the monitored pressure changes by more than a specified amount.

6. The method of claim 3 , wherein the maintenance action includes at least one of:

re-tightening at least one of the packing seals;

replacing at least one of the packing seals; and

replacing the valve.

7. The method of claim 3 , wherein the pressurizing fluid is nitrogen gas.