Column VentDetector

Carrier gas Inlet

Sample Inlet

: Plunger Down

: Plunger Up

: Plunger Down

: Plunger Up

Vent

Carrier gas Inlet

Sample Inlet

Column Detector

Things you should know about GC diaphragm valve

Flow mixing, what is this? When it is not desirable? When it is desirable to have it ?

TODAY, AFP GC DIAPHRAGM VALVES ARE MORE FREQUENTLY SPECIFIED

THAN THE ONE DEVELOPED YEARS AGO. THANK TO THE TECHNICAL

IMPROVEMENTS DONE BY AFP, NOW THESE VALVES ARE REALLY

RELIABLE AND ARE OFTEN USED TO REPLACE ROTARY VALVES IN GAS

CHROMATOGRAPH. THEY ALSO PROVIDE SELF DIAGNOSTIC FEATURE,

A WORLD PREMIER. IN ORDER TO GET FULL BENEFIT OUT OF THEM, SOME

TECHNICALS DETAILS MUST BE TAKEN INTO CONSIDERATION FOR SOME

SPECIFIC APPLICATIONS. THE PURPOSE OF THIS PAPER IS TO PROVIDE

THE INFORMATION NECESSARY TO UNDERSTAND THESE TECHNICAL

DETAILS AND POSSIBLE IMPACT ON YOUR SYSTEM.

Flow mixing is a phenomenon that occurs in a GC diaphragm valve upon actuation. For a short period of time, all the ports of the valve become in communication, i.e. all ports are open at the same time. This means that any high pressure stream can flow back in any lower pressure port or stream. Figure 1 shows the simplest gas chromatographic configuration. Imagine a liquid sample at a pressure of 500 psig, or even higher, flowing into the sample loop, being injected into carrier circuit to be allowed to be separated by the gas chromatographic column.

GC DIAPHRAGM VALVE SERIES APPLICATION IDEAS

Mixing or not mixing ?

FIGURE 1 : The simplest GC configuration

Sampling

Injection

AB-04Application Brief-04

2

TIPS INFO :Here the mobile phase or carrier is gas while sample is liquid. This is a common situation in gas chromatography. Do not confuse this situation with liquid chromatography i.e. HPLC. Many hydrocarbon samples will vaporise or evaporate at the atmospheric pressure. It is therefore required to maintain them at higher pressure to avoid sample fractionation or distillation, leading to inaccurate measurement. The most volatile compound will vaporise when the heavier or less volatile ones will still be in liquid phase. Such sample under high pressure condition must be quickly injected into the carrier circuit as a “slug” and then vaporised in the gas phase. This is done by allowing the sample liquid “slug” to flow through a heated zone. When the liquid sample is vaporised, its volume will increase anywhere from 300 to 900 times its original volume. For this reason, the sample loop size is very small to take this phenomenon into account. A larger liquid sample volume will generate a too high volume of gas sample after vaporisation step, causing separation column overloading.

FIGURE 2 : Typical actuation step for commercial valve FIGURE 3 : Actuation step but for AFP valve

Now coming back to Figure 1, imagine during sample injection step i.e. when introducing the sample loop volume into carrier circuit, that for a short period of time during that process, all valve’s ports are open because all valve’s plungers are retracted.

This situation will allow the high pressure liquid sample to flow back into low pressure carrier circuit and into column inlet. These parts of the system become momentary connected to the high pressure sample source. This will have a negative impact on system performance, since the volume injected will not be repeatable and also larger dependent on actuation time, that itself depends of the pneumatic actuation line and solenoid valve size and CV. Furthermore, depending on sample vapor pressure partial, fractionation will occur during this process. Figure 2 shows typical plungers displacement related to the actuation pressure rise, for typical GC diaphragm valve available on the martket.

When used for such application, our valves are generally tuned to have a dead band of 15 psig, when the pressure rises into the actuator. This is done with the help of pressure adjusting screw mounted in the center of bottom cap. This screw is not used to hold the cap on the cylinder body like the valve found generally on the martket. Dead band zone may be defined as the zone where all plungers are up for a certain actuation pressure range. This value is adjustable between 5 to 15 psig. Generally the valve dead band is set at 15 psig, as show in Figure 3. This will prevent any flow mixing or cross port flow into the valve, improving system performance when operating under these conditions. It maintains the ‘‘slug’’ type injection. In such application, flow mixing is not desirable.

3afproducts.ca

TIPS INFO :One of the differences between a diaphragm and rotary valve is the method used to divert the flow between ports. The rotary valve is a volume displacement valve i.e. to control the flow direction, a volume is aligned to allow flow path between two adjacent ports. This space or this volume is displaced, generally between two positions. This way, there is no dead volume because this volume, or more precisely a groove machined on a rotor surface, is precisely put in line with the two adjacent ports.

Things are different for a GC diaphragm valve. Indeed the flow direction is controlled not by introducing a volume between two ports, but by closing or reducing it, interrupting the flow path, as shown in figure 4. For this reason, mixing could occur only in a diaphragm valve.

FIGURE 4

When a controlled level of flow mixing is desirable ?

When sampling at the atmospheric pressure and the sample volume is small i.e. about 100 microliters or less, it may be desirable to retune the valve to get a little bit of flow mixing upon actuation. This will effectively eliminate the dead volume effect generated by the space between the plungers and their adjacent ports.

P5 P6

P4 P1

P3 P2 PURGEIN

PURGEOUT

P5

P4

P6

P1

P3 P2

Groove (Displacement Volumeused to connect two ports)

Coated drive/bushing adaptor

Washer

Preload ass’yhousing

Dowel Pin

Mounting collar

Stator

Hardened thrusttransfer disc

Locking screw

Washer

Pressure adjustingscrew with

fine pitch thread

Preload assembly

Treatedrotor

Ball bearing

Spring(Temperature compensated)

Diaphragm ValveRotary Valve

4

Referring to figure 5, we can see where those small dead volumes are located. These small dead volumes could in some situation, like mentioned here above, generate a second, but with much less sample volume, injection.

Indeed, when operating the G.C. diaphragm in a non mixing mode and the sample volume is small this phenomenon could appear. It is all dependent of sample solubility and diffusion speed. It will be much more apparent with small liquid sample than gas, this is due to volume gain upon liquid sample voparisation. The sample pressure and temperature also plays a role. So, in short, this phenomenon shown in figure 6 may not appear at all. When the valve of figure 5 is actuated and injecting the sample into carrier circuit, there could be an accumulation of sample fluid in the dead volume DV-1. This volume is not place directly into the carrier flow path but may diffuse slowly or rapidly into the carrier.

FIGURE 5

FIGURE 6

A - READY TO INJECT FILLED SAMPLING LOOP

C - ALMOST COMPLETELY INJECTED

B - WHEN ACTUATED

D - WHEN SWITCHED BACK TO REFILL SAMPLING LOOP. DV1 IS PARTLY FILLED WITH SAMPLE, WHICH WILL CREATE A SECOND INJECTION

DV1Vent

Carrier gas inlet

Sample InletPlunger Down

Plunger Up

Vent

Carrier gas inlet

Sample InletPlunger Down

Plunger Up

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

DV1

Column Detector

Column Detector

DV1Vent

Carrier gas inlet

Sample InletPlunger Down

Plunger Up

Vent

Carrier gas inlet

Sample InletPlunger Down

Plunger Up

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

DV1Column Detector

Column Detector

DV1Vent

Carrier gas inlet

Sample InletPlunger Down

Plunger Up

Vent

Carrier gas inlet

Sample InletPlunger Down

Plunger Up

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

DV1

Column Detector

Column Detector

DV1Vent

Carrier gas inlet

Sample InletPlunger Down

Plunger Up

Vent

Carrier gas inlet

Sample InletPlunger Down

Plunger Up

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

DV1Column Detector

Column Detector

0.0

0.80

0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

1.02

1.04

mVo

lts

0.80

0.82

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0.86

0.88

0.90

0.92

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0.98

1.00

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1.04

mVo

lts

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

injection

O 2 N 2 CH 4

Switch Back V1

Re-In

ject

ion

O 2

N 2

CO

H 2

5afproducts.ca

Now, when the valve is restore to its original position, this volume will be swept by the carrier and introduce into the carrier circuit, generating a second injection and a set of twin and smaller peaks. This issue could be resolved in two ways. The first solution it by choosing the right time to set back the valve in sampling position in order to get this set of small twin peaks between the real ones, and samply ignoring them. It hurt nothing to do so.

The next method is to allow a controlled mixing upon injection. This could be done at the factory or by the end user, after reading proper instruction sheet about this. Allowing a little flow mixing will force the dead volume to be swept by the carrier gas, moving the accumulated sample away through separation column. This way, when the injection valve is returned to sampling position, there will be no second injection. Figure 7,8 and 9 are showing what happens in such case.

FIGURE 7 : The simplest GC configuration

Column

: Plunger Down

: Plunger Up

VentDetector

Carrier gas Inlet

Sample Inlet

: Plunger Down

: Slightly leaking Plunger

: Plunger Up

Vent

Carrier gas Inlet

Sample Inlet

Column Detector

Column

: Plunger Down

: Plunger Up

VentDetector

Carrier gas Inlet

Sample Inlet

: Plunger Down

: Slightly leaking Plunger

: Plunger Up

Vent

Carrier gas Inlet

Sample Inlet

Column Detector

Sampling

Injection

FIGURE 8

DV1Vent

Carrier gas inlet

Sample InletPlunger Down

Plunger Up

Vent

Carrier gas inlet

Sample InletPlunger Down

Plunger Up

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

DV1

Slightly leaking Plunger

Slightly leaking Plunger

Column Detector

Column Detector

DV1Vent

Carrier gas inlet

Sample InletPlunger Down

Plunger Up

Vent

Carrier gas inlet (P1)

Sample Inlet (P2)

Plunger Down

Plunger Up

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

DV1

Slightly leaking Plunger

Slightly leaking Plunger

P1 P2

Column Detector

Column Detector

DV1Vent

Carrier gas inlet

Sample InletPlunger Down

Plunger Up

Vent

Carrier gas inlet

Sample InletPlunger Down

Plunger Up

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

DV1

Slightly leaking Plunger

Slightly leaking Plunger

Column Detector

Column Detector

DV1Vent

Carrier gas inlet

Sample InletPlunger Down

Plunger Up

Vent

Carrier gas inlet (P1)

Sample Inlet (P2)

Plunger Down

Plunger Up

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

P1

P2 P3

P4

P5P6

PL6

PL1

PL2

PL3

PL4

PL5

DV1

Slightly leaking Plunger

Slightly leaking Plunger

P1 P2

Column Detector

Column Detector

A - READY TO INJECT FILLED SAMPLING LOOP

C - TRANSIENT STATE UPON INJECTION THAT ALLOWS TUNE-MIXING. THIS STEP OCCURS UPON ACTUATION

B - WHEN ACTUATED

D - WHEN SWITCHED BACK TO REFILL SAMPLING LOOP, DV1 IS NOW FILLED WITH CARRIER GAS

6

FIGURE 9

0.0

0.80

0.82

0.84

0.86

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0.92

0.94

0.96

0.98

1.00

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mVo

lts

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0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

1.02

1.04

mVo

lts

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6

injection

O 2 N 2 CH 4

Switch Back V1 No

seco

nd In

ject

ion CO

H 2 If required and if the application is critical, AFP could provides specific valve and plunger assemblies (patented technologies). These plungers could be actuated at different speed or at different moment in time during the actuation sequence. Such arrangement makes sure that carrier gas always sweeps the dead volume space DV1 at all time.

TIPS INFO :

Port pressure drop

One characteristic that differs the diaphragm valve from a rotary valve is the port pressure drop between the two valve operating states, i.e. normally open sections in the unactuated state and normally closed sections when the valve is actuated.

In the rotary valve, the pressure between the two rotary valve operational positions is the same. Indeed, as previously said, a volume is displaced when the valve when the valve is switched between positions. The volume in the groove machined into the rotor surface is always the same whatever is the valve position. So is the pressure drop.

In a diaphragm valve, things are different. The diaphragm is lifted up against the valve head in order to interrupt the flow between two adjacent ports. When the plunger is retracted, the diaphragm will take back its place into the valve actuation body groove in order to restore the flow.

Normally, the diaphragm memory effect is enough to reseat it. Furthermore, the line pressure will help to re-seat the diaphragm and clear the flowpath between ports. However, there are some situations where the diaphragm will not re-seat completely causing a difference in pressure between valve positions.

1 At high temperature, the diaphragm mechanical behaviour changes and the diaphragm loses some of its spring back capacity.

2 Operating at low or sub-atmospheric pressure (not vacuum), for example when pulling the sample through the valve with the help of a sample pump, the diaphragm will have a tendency of being lifted of a little causing a slightly higher pressure drop.

3 When working with capillary columns, the column head pressure is very low and in some cases almost equal to the valve pressure drop. However the velocity is relatively high. So, in such case, any slight variation of valve port pressure drop upon actuation will affect carrier flow velocity into the column.

Most of the time, the slight pressure drop variations between valve positions is not an issue and doesn’t affect system performance at all.

However, if required, it is possible to cancel this phenomenon. Figure 10 shows how to reduce flow perturbation upon actuation by increasing the ‘‘impedance’’ of the source of carrier. During sampling mode, the valve pressure drop for the carrier flowpath is equal to ΔPA. During sample flow injection, this value is now ΔPB + ΔPC + ΔPSL, so a higher pressure drop or restriction is put in series with the column.

FIGURE 10

Capillarycolumn

Vent

Vent

Detector

Carrier gas Inlet

Sample Inlet

PR1∆PA

∆PSL

R1P1

∆PC

∆PB

This extra pressure drop or flow restriction may reduce column head pressure and carrier velocity through it. This is because the valve pressure drop is close to the column pressure drop. It’s like adding two restrictions almost of the same value is series.

Now by adding a restriction R1 between the valve and the column, and tuning R1 in order to get the required flow through the column at a substantially higher pressure value P1 will cancel any negative impact of the slight pressure drop change upon valve actuation, or a temperature change like in a temperature programmed environment.

Typical value for P1 has been set from 30 to 90 psig. This system configuration allows also monitoring the carrier flow by cross-referencing it to pressure P1. It is easy to set up a look-up table in the software to do so.

We did test this configuration with great results. No velocity change at all through the column, so peak timing and shapes unaffected at all.

FIGURE 11

Capillarycolumn

Vent

Sample Vent

Detector

Carrier gas Inlet

Sample Inlet Sample ventSample pump

PR1

∆PA

∆PSL

P1

P

∆PC

∆PB

R1

Or

Figure 11 shows how to eliminate problem on the sample side of the system. In some reactor or catalyst performance monitoring, it is really important to not modify any operating parameters when sampling the effluent. This necessitates maintaining the flow and sampling pressure of the system constant.

The best way to achieve that is by adding a miniature sample pump in the sample vent of the valve. A miniature by-pass control valve could be added to maintain the pump inlet pressure at a constant pre-set value. This is achieved with an inexpensive pressure sensor and an independent PID controller. All this costs less than 300$ and performs exceptionally well.

In the case that the sample is higher than the atmospheric pressure, a restriction R1, as shown in Figure 11 could be added. By properly tuning R1 relatively to P1, the internal part of the valve and the sampling loop are maintained at a constant higher pressure. This eliminates the effect of any slight variation in valve pressure drop. Sample flow will be constant during injection and sampling.

Same philosophy could be followed when using the valve for a column switching application. Slightly increasing column pressure or back pressure seen by them will improve in such application the performance of the system.

ConclusionMost of the time, this slight changes in pressure drop doesn’t affect at all system performances. Knowing diaphragm valve behaviours make it easy to correct them in case of impact on your system. This is application dependant.

ANALYTICAL FLOW PRODUCTS AN APN GLOBAL COMPANY - 2659, BOUL. DU PARC-TECHNOLOGIQUE, QUÉBEC, (QUÉBEC) G1P 4S5, CANADA - INFO@AFPRODUCTS.CA - afproducts.ca *See disclaimer notice available on our website. *Wrote by Yves Gamache, former AFP CTO. ©Copyright 2015 Analytical Flow Products