As the world continues to shift towards decarbonization in chemical production, and strong market demand drives the transition to carbon-free energies, ammonia’s role in the green energy economy continues to expand. In Black & Veatch's recently release Hybrid LNG & Ammonia Infrastructure: Key to a Green Economy eBook, we explore cost saving design solutions to prepare for a transition to carbon-free energy.
From that eBook, this section discusses the optimum conversion of existing LNG storage tanks to ammonia storage tanks.
Design Parameters
The chemical properties of both LNG and anhydrous ammonia need to be considered when preparing to convert an LNG storage tank to an ammonia storage tank (Table 3).
It is also critical to adhere to storage tank design codes including LNG: National Fire Protection Association (NFPA) 59A, American Petroleum Institute (API) 625, API 620 Annex Q and Ammonia: American National Standards Institute (ANSI)/CGA G-2.1, API 625, API 620 Annex R.
Typical LNG Tank Configurations
Typical LNG tank types are double wall, single-containment tank systems or full-containment tank systems. Membrane-containment tanks are currently used in Asia, notably Japan, Korea and Taiwan. Membrane-containment tank systems are typically built in-ground with the main body of the tank (except the roof) buried under ground level. The membrane-containment tank system is not considered in this paper because its design considerations are quite different from single- or full containment tank systems. Figure 3 (API 625: 2018) shows the configuration of the double-wall, single-containment tank system.
The main materials for double-wall, single-containment LNG tanks are listed in Table 4.
Figure 4 (API 625: 2018) shows the configuration of the full containment tank system.
The main materials for the full-containment LNG tanks are shown in Table 5.
Materials
The materials used for full containment and single-containment LNG tanks are generally compatible with refrigerated ammonia tanks. However, under certain conditions, liquefied ammonia is known to cause stress corrosion cracking (SCC) in steel. Extensive research and investigations have been done on ammonia SCC, leading to preventative measures being adopted on low-temperature carbon steel used for refrigerated ammonia tanks.
But to date, very little research has been done on ammonia SCC with 9-percent Ni steel – with this in mind, further research and investigation will be required to ensure that the structural integrity of the tank is maintained throughout the design life.
Structural Design of Tank
In the structural design of LNG tanks, design liquid level and liquid density are the necessary parameters used to calculate the static and dynamic liquid pressures acting on the wall and foundation. The structural integrity of the tank must be revalidated using the increased hydrostatic load. Based on the ratio of LNG density versus liquid ammonia density, maximum liquid level allowed for ammonia storage is expected to be approximately two-thirds of the original design, hence the nominal tank capacity when used for ammonia will be approximately two-thirds of the original design.
Insulation System
Insulation system for a typical LNG tank between 100,000m3 and 200,000m3 capacity is designed to meet the BOG rate of less than 0.05 percent per day. Due to ammonia’s higher boiling point and heat of vaporization, the BOG rate while storing ammonia will be significantly less than the rate while storing LNG.
Tank Accessories and Appurtenances
Tank accessories and appurtenances must be compatible for both LNG and ammonia and should be replaced when the stored liquid is changed.
- In-Tank Pumps – In-tank pumps will be required to be replaced when the stored fluid is changed from LNG to ammonia, or ammonia to LNG.
- Tank Instrumentation – Level gauges and density gauge require replacement or modification. Alarm settings for leak detection temperature gauges, etc., requires adjustment.
- Pressure Relief Valve (PRV) – PRVs equipped for LNG use are required to be replaced with PRVs sized for ammonia.
Decommissioning and Re-Commissioning
The LNG tank will need to be decommissioned before it is converted to an ammonia tank. The decommissioning process involves tank emptying, isolation, warmup and tank inerting (nitrogen purging), although detailed procedures should be established on a case-by-case basis.
Research on dismantled LNG tanks show no evidence of unsatisfactory structural or operational performance after 20+ years of operation. Depending on years of operation and the operational history of the tank, it should be evaluated whether tank entry and internal inspection or repair work would be required.
The procedure to re-commission the tank is similar to the initial commissioning of the tank. The re-commissioning process involves the testing of tank accessories, tank cooldown and initial filling.
Looking Ahead
Further study is required on the effect of SCC behavior of 9-percent Ni steel in ammonia. Once its effect and prevention methods are confirmed, double-wall, single containment LNG tanks and full containment LNG tanks can be converted to ammonia tanks as described above.
About the Author
Hidemichi Iizuka is an Engineering Manager with Black & Veatch’s Oil & Gas business. He has more than 25 years of experience in largescale cryogenic storage tank design for multiple international engineering, procurement and consulting (EPC) projects. Knowledgeable in international design codes, Iizuka specializes in the design of various types of storage tanks and field erected tanks. He is a member of the American Petroleum Institute (API) Refrigerated Tanks Task Group and Membrane Tanks Working Group.
