08Downstream

What leaves the outlet

Downstream design starts from these figures. All of them are without a dryer attached.

What to decide first

The downstream is not a choice between methods but a stack of layers built to a requirement. Fix what the application demands as numbers first, and the number of layers follows.

1. Write down the dew point and purity the application requires, as numbers.
   • Use the figures from the specification if you have one; if not, tell us what it is for. The application alone is usually enough for us to narrow it down with you.
2. Write down the pressure you need.
   • If the pressure at the point of use differs from the storage pressure, note both. This decides whether you need a compressor at all, and whether to run the system at 0 or 10 barg.
3. Decide whether you store it or use it as it is made.
   • Storing brings compression and vessels with it, plus the overpressure protection and permitting the code requires. Using it as produced turns the question into whether the application tolerates a varying flow.
4. Decide how many units and add up the flow.
   • Downstream equipment is selected on the combined flow. One unit is 0.48 Nm³/h, and if you plan to add units later it is better to size for that from the start.
5. Then build the layers.
   • An application that only needs a lower dew point stops after one or two layers. One that sets a purity target adds the later ones.

Attaching drying and purification

Scale is the first obstacle. A single HXB-V1 produces 0.48 Nm³/h. General-purpose industrial hydrogen dryers typically start around 5 Nm³/h, and even the small units built for electrolyzers stop at about 1 Nm³/h. Products that fit this band are scarce, which is why we are building the HXD-V1 modular dryer.

What to check

• Whether the rated flow covers this scale. A model with a minimum flow will not run properly at 0.48 Nm³/h.
• The inlet pressure it needs. Dryers that run at ambient and dryers that assume a pressurised feed are different machines.
• The pressure drop. That figure becomes backpressure at the system outlet.
• How it regenerates. Pressure-swing regeneration vents some product gas each cycle, and at a small flow that loss weighs more.
• Consumables and their intervals. Adsorbents and getters are parts you replace when they are spent.
• Certification in the country of installation. Establish first whether the location calls for an explosion-proof rating.

Compression and storage

The compressor sets what has to come before it. Each one has a minimum suction pressure, and if the upstream cannot deliver it the compressor will not run. This is the real reason to choose between 0 and 10 barg on the system.

What storage decides

Overpressure protection

When you connect the hydrogen outlet to downstream or storage equipment, provide overpressure protection to your code and design. The protections inside the system do not protect your equipment.

Permitting and installation

High-pressure gas permitting and site installation stay with the site owner. What we can handle also changes with the storage pressure and capacity.

The manufacturer's warranty

Compressors and vessels we source come with the manufacturer's warranty unchanged. We also carry the overseas purchase and customs.

Do not create backpressure

This is the most common problem introduced by adding a downstream. Narrow or block the path out and the pressure pushes back into the system.

• Fit no valve and nothing that can block on the vent and relief lines. They have to stay open.
• Do not run with a downstream valve closed. Confirm the path is open before start-up.
• Do not kink or pinch the tubing, and keep it free of debris and residue.
• Put a drain or water trap at the low point. The outlet gas is saturated and pooled water will block the line.
• Watch for freezing. Vent and relief lines holding condensate can freeze shut. Use slope, drain points, insulation or heat tracing as your site requires.
• Keep objects away from in front of the vent outlets.

What to measure, and where

There is no hydrogen purity analyser inside the system. Downstream quality has to be measured downstream, and where you measure decides the number.

The oxygen outlet

This is the side people forget when planning a downstream. Splitting water produces oxygen too, half the hydrogen by volume.

• Route O2 OUT to its own safe, well-ventilated discharge point.
• Never combine it with a hydrogen line or vent. It needs a separate discharge point.
• An oxygen-enriched atmosphere raises the fire risk. Keep combustibles away from the discharge area.
• The oxygen stream is saturated too. Keep water from pooling at the low points.

The boundary, once more

Everything in this chapter belongs to your site. We can source it for you, but the design responsibility and the permits stay with the site owner.

What the system does

It delivers hydrogen to H2 OUT at the selected pressure and flow, and relieves pressure through H2 VENT in abnormal conditions.

What your site does

Vent and exhaust routing, drying and purification, compression and storage, the connection to the point of use, downstream instrumentation, and whatever overpressure protection and permits the code requires.

Where we can help

Give us the conditions and we will settle the configuration with you and source it. We also carry the overseas purchase and customs.