When selecting valves for ammonia piping, the single most important point for preventing corrosion trouble is material selection. In short, because ammonia attacks copper and copper alloys (brass, bronze, and the like), valves containing these materials should be avoided, and selection should be based primarily on carbon steel, stainless steel, and cast iron systems. Furthermore, corrosion risk changes with the actual fluid conditions—whether the ammonia is dry or wet, in the liquid phase or gas phase, and at what temperature and pressure—so it is necessary to verify not only the metal body but also the compatibility of sealing materials such as gaskets and packing with equal weight. Here, we organize the criteria for preventing corrosion in the order they arise in practice.
Key Points of This Article
- Because ammonia attacks copper and copper alloys (brass, bronze, phosphor bronze, etc.), the general rule of thumb is to avoid these and select primarily from carbon steel, stainless steel, and cast iron systems.
- Dry ammonia and wet ammonia containing moisture behave differently, and the coexistence of moisture and oxygen can become a factor of concern for stress corrosion cracking.
- Selection is only complete once you have confirmed not only the metal material but also the compatibility of sealing materials (gaskets, packing, diaphragms).
How Ammonia Affects Valve Materials
Why Copper and Copper Alloys Should Be Avoided
Ammonia reacts readily with copper and copper alloys, and when moisture or oxygen coexist, it can lead to corrosion or stress corrosion cracking. This applies not only to pure copper but also to alloys whose main constituent is copper, such as brass, bronze, and phosphor bronze. Copper alloys may be used not only in the valve body but also in trim and internal parts such as the seat, disc, spring, and gland section, and care is needed because this is difficult to judge from the external appearance. The starting point is to check the material of the wetted parts in specification sheets and catalogs and to select a configuration that contains no copper alloys.
Corrosion Risk Varies with Dry/Wet and Liquid/Gas Phase
Even with the same ammonia, corrosion behavior differs between dry ammonia containing almost no moisture and wet ammonia containing moisture. Generally, it is said that when oxygen coexists in a wet condition, stress corrosion cracking may be a concern even for carbon steel. The ammonia refrigerant (R717) used in refrigeration and air conditioning is often used under relatively dry conditions, whereas process applications may involve moisture. Selecting materials after organizing whether the fluid is in the liquid phase or gas phase, the operating temperature and pressure, and the presence of trace moisture or impurities improves the accuracy of the decision.
How to Select Compatible Materials and Sealing Materials
Approach to Body and Trim Materials
For ammonia applications, iron and steel systems such as carbon steel (including cast steel and forged steel), stainless steel (SUS304, SUS316, etc.), and ductile cast iron or cast iron are the basic options. Steel valves are commonly used in ammonia refrigerant equipment for refrigeration and air conditioning. Stainless steel has high corrosion resistance and may be selected under conditions where the influence of moisture or impurities is a concern, but the guideline is to select it appropriately—neither more nor less than needed—in light of the application, pressure, and cost. In every case, along with the body material, verify that the internal trim contains no copper alloys.
Compatibility of Non-Metallic Materials Such as Gaskets and Packing
Even if the metal material is selected appropriately, if the sealing material is attacked by ammonia it will lead to leakage or degradation. Non-metallic materials such as gaskets, packing, and diaphragms must have their compatibility with ammonia confirmed for each material. Generally, PTFE and elastomers compatible with ammonia are used, and care is needed because suitability can differ depending on the material. Since the range of compatibility also changes with the operating temperature and the liquid or gas phase, the reliable approach is to confirm after presenting the actual fluid conditions.
Frequently Asked Questions
Q1. Can brass or bronze valves be used in ammonia piping?
A1. As a general guideline, they should be avoided. Ammonia can attack copper alloys, and the coexistence of moisture and oxygen may increase the risk of corrosion or cracking. Please consider iron and steel materials as the basis.
Q2. Which should I choose, carbon steel or stainless steel?
A2. It depends on the conditions. For dry refrigerant applications, steel is common, while stainless steel may be selected under conditions where the influence of moisture or impurities is a concern. The decision is made in light of the operating conditions and cost.
Q3. Are the material precautions the same for a refrigerator’s ammonia refrigerant (R717)?
A3. The principle of avoiding copper alloys is common. Although refrigerant applications are often under relatively dry conditions, the conditions change depending on the presence of moisture, oil, or impurities within the system, so it is safest to confirm under actual operating conditions.
Q4. What should I choose for sealing materials (packing, gaskets)?
A4. Select a material whose compatibility with ammonia has been confirmed. PTFE and compatible elastomers are often used, but because suitability differs by material, please confirm including temperature and phase state.
Q5. What should I watch for when replacing an existing ammonia valve?
A5. Rather than searching for an equivalent product by model number alone, check whether the fluid conditions (dry/wet, temperature, pressure) have changed between then and now. If there have been operational changes, reviewing the material and sealing material at the time of replacement improves accuracy.
Q6. What should I do if I cannot determine whether moisture is present?
A6. First, confirm the actual operating and maintenance situation of the system with the operations side. If the basis for judgment is not available, the reliable approach is to organize the conditions, make use of a technical consultation, and consider a material selection with a margin of safety.
Summary
Selecting valves for ammonia piping starts from avoiding copper and copper alloys and placing iron and steel systems at the core, and is only complete once you have confirmed the fluid conditions such as dry/wet and liquid/gas phase, as well as the compatibility of the sealing material. In all cases, starting from a “general guideline” and then applying it to the actual temperature, pressure, and moisture conditions of your own equipment leads to the suppression of corrosion trouble.
If you have concerns such as “I want to select an equivalent product that contains no copper alloys,” “I cannot tell whether the ammonia is dry or wet and cannot finalize the material,” or “I want to replace an existing valve but the operating conditions have changed,” please organize your fluid conditions and make use of Yoshitake’s technical and selection consultation. Based on your application and fluid conditions, we will work together to consider valve selection with suitable materials and sealing materials.